The Business Week article mentioned in the post is old, but has some interesting points, like:
But young consumers may follow brands offline -- if companies can figure out how to talk to youths in their online vernacular. Major companies should be exploring this new medium, since networks transmit marketing messages "person-to-person, which is more credible," says David Rich Bell, a marketing professor at the University of Pennsylvania's Wharton School.snip
In fact, the advertising can be so subtle that kids don't distinguish it from content. "It's what our users want," says Anderson.
"Kids don't buy stuff because they see a magazine ad. They buy stuff because other kids tell them to."
Just replace the buy with try on that last one if you think it doesn't apply.
But I'm not sure that just putting up a page on MySpace or Facebook is enough. Maybe there a small amount of residual "cool factor" that you get if someone stumbles on your page (yet how do they find you and why would they want to visit?), but otherwise just being there isn't enough. You need to add value, provide an extra service, respond right when they need it, etc. (See more from Jill Stover about this). And that can be hard and time-consuming to do. Mass marketing is time efficient, but less and less effective.
The Ubiquitous Librarian (great blog!) has some ideas and experience with using social networking sites for niche marketing. These are the kind of reasons to be involved with social networking sites that make sense.
Niche marketing opportunities go beyond social networking sites. Have you set up alerts and feeds to monitor what people say about your library online? The small investment of time is well worth it on this one. You'll be amazed at what you find. And if a complaint, rant or problem a user had winds up in your aggregator do you take the next step and contact them and solve it?
I'm far from an expert in any of this, but I think the basics best practices with any promotion effort apply. Focus on benefits to the user (no one cares about your new service or product by itself, they want to know what it will do for them), be interesting (or better yet be remarkable) and to hit a home run, give them something to talk about.