First, I should warn you that I have opinions here. They're a little different from what you might expect, though. (Also, please bear with me until I get to the actual focus of this Thing).
You see, I spent 18 months (from early 2003 through fall of 2004 as a volunteer for a different online site (LiveJournal.com) as a member of their Abuse/Terms of Service team. I found it fascinating and compelling (if sometimes very hard) work: the team handles concerns about the Terms of Service, so it covers everything from Digital Millenium Copyright Act requests to harassment problems to "Can I sell things on my journal?" to what happens when a 16 year old posts provocative mostly unclothed pictures of themselves to people posting suicide notes.
Needless to say, it gave me a very interesting perspective on online networking sites, how they're used, and how widely varied the assumptions of private or public access can be.
And, on totally other levels: I've seen it save lives. (I've been a part of that, a couple of times.) I've seen people get help after fires and other tragedies. I've seen endless helpful, meaningful, patient support and good ideas. I've seen people who have felt constantly friendless and like total outsiders find places where they felt comfortable expressing themselves, growing, and being seen as thoughtful contributors to communities they care about.
LiveJournal is how I knew on 9/11 that friends in the NYC and DC areas were safe, and it's meant getting local information, unfiltered by news reports, about what it's like for everything from 9/11 to missles from Israel into Palestine a few years ago (from a friend in Israel). While I've also seen online interactions cause a lot of hurt (through harassment, etc.), the good still massively outweighs the bad for me.
It's because of these last two paragraphs that I believe that *all* online communication has the potential for good things - and that barring issues like very limited resources that are needed for school work or the specific goal for machines (as may be the case in schools), people may be using social networking sites for many of the same reasons older people might be using email - to keep in touch with friends, to learn new things, and to share what they know and feel.
What does this mean?
I bring this up, because I think it's an interesting lens on how people approach online sites. Some people want to make connections with others on a fairly superfical level: they have one thing in common, and share conversation about that, and it's good.
Some people run deeper: I have several very good friends I met doing Abuse Team work: people I would happily (and have happily) put up in my home, would make plans to travel to see (budget allowing) and would gladly do what I could to help. I also have a number of people on my Friends List on LiveJournal who I'm less close to - but who reliably say things that make me think in new ways, or who challenge poorly-thought-through conclusions.
It's worth noting that the code of different sites makes different kinds of communication more or less appealing. LiveJournal's initial focus as a journaling site (with longer posts, shared communities, etc.) has meant that it's attracted people who want to have longer in-depth conversations, as well as those who wanted to use it for shorter or less detailed ones. Facebook and MySpace's designs both tend toward shorter, more limited posts: these are great for some kinds of conversations, but not for others.
Likewise, site design can have a huge impact on people: there's some evidence that they're attacting different communities, and certainly, individual people have preferences. (I have a very hard time on MySpace, because I strongly prefer visually simple interfaces and structures in my online space. And while I find Facebook more visually pleasing, the shorter length of communications is not my preference).
Power of combined knowledge:
There's also a fascinating other power, that's been around on social online interactions for, well, decades. I first saw it expressed as "All Knowledge is Contained in Fandom" (from the science fiction fandom community) and as "All Knowledge is Contained on Usenet" (the old-fashioned text-based communication systems that had its real hey-day in the 1990s). These days, I regularly see posts tagged "AKICILJ" (All Knowledge is Contained in LiveJournal)
The point is - you make a post (usually with the relevant tag in the subject line.) You ask your question. People respond. (The AskMetaFilter site is, more or less doing the exact same thing - except that there, you have a particularly broad cross-section of expertise.)
What have I had answered there? All sorts of useful questions.
When we were looking to subscribe to a SF or fantasy magazine, I asked which ones tended to have covers that would be the best fit for a high school (we have Sports Illustrated, and People, and several other titles, but still, it's nice to know what might show up on the cover.) I've asked questions about books that I can't track down in other ways. I've asked for ideas with book displays or purchases.
I've also made use of community discussions (LiveJournal, like most networking sites, has several library-related communities) a number of times (and commented on them far more: there are questions about job hunting and specific library situations and suggested titles, on a regular basis.)
Back to the actual focus of this Thing, though..
Do I think libraries should be on Facebook and MySpace?
I think it depends a lot on the library. I think it also depends on upkeep. Keeping up an online conversation or interaction involves time and energy: just creating an account isn't good enough - people will lose interest or forget you're there. At the same time, many people (especially those not familiar with a range of online cultures) think that More Information, All The Time is the way to go. Fact is, that's a good way to really turn people off.
In other words, I think that if you're going to do it, do it right. Take time to look at other successful uses (both libraries and other settings). Learn a little about likely cultural issues.
One thing to understand about the 'Net is that it largely grew up as a system of interconnected personal projects. There are exceptions, and some sites grew into major corporate endeavors very fast. Some sites have certainly taken to advertising like fish to water - but in many cases, there's still quite a bit of tension about what advertising is okay for users, and what advertising or sponsorship isn't.
The same goes for tools like poking or sending virtual gifts. Will kids at your school or in your community be weirded out if you friend them back? The culture matters to a lot of users, even when they can't always explain exactly what the rules or comfort zones are. Different sites have different cultures, and different etiquette, too. (As do sub-communities within a particular site: the people I hang out with on LiveJournal have different etiquette than many teenagers on the site, for example, especially about adding or removing people from a friends list, or about how to balance relationships.)
Figure out what specific content you want to share, and whether a given site is the best way to do it. Maybe you want to post 3 new titles each week with a link to a list of other items - but gear it to your likely audience. Maybe you want to have your YA librarian have their own account for your library, so you can focus even more closely. And so on.
I definitely think we - as librarians, and especially in the schools - should be pushing for ways to include more education and information and resources about how to use online sites. Not in a heavy handed safety way (though some of that information is important) but in the sense of being a resource for getting initial help for someone who's concerned about harassment issues, or who is unsure if something's a scam, or any number of other issues. I've done some work on this at my current job, and I'm interested in doing a great deal more. (Check out the links from my website.)
A list of links on a library website (to friendly, appropriate info sites) is one way to go. But working that information into other workshops and programming, or making sure you have books in stock so parents can read and learn about the topic is also important. For recent presentations, I've been looking at materials on online safety. Minneapolis has some, but not others. Ramsey County has some but not others. And so on - it can be a pain to get books with a specific focus or viewpoint at times. Making the information easier to get is definitely within a library's scope. Publish Post
One more resource I want to mention, before finishing this very long post is danah boyd, who is currently finishing her dissertation focusing on online networking sites. She makes fascinating posts about online sites, who uses them, and many surrounding issues, from the perspective of someone who uses and loves these sites herself. http://danah.org (starting with the 'popular essays' is a good way to go.)