I had fun with this thing -- creating a facebook page immediately put me back in touch with some old friends, it was incredibly interesting to see the profiles of my nephews who are currently in college, and the widgets/quizzes/etc were fun little pop things to do (my dragon currently weighs 274 pounds and has accumulated 52 gold coins).
Leaving my personal fun out of it ... I looked at individual library pages on both Facebook and MySpace. The MySpace pages were very much geared towards teens; so much so that I thought a couple of them would be off-putting to anyone over the age of 23. The Facebook pages seemed to be more age inclusive.
Then I took a look for what might work best for my library. I wrote down a list of 20 teens who regularly visit us and checked both sites to see if they were listed. The results were as follows: 10 were on neither site, 2 were on both sites, 5 were only on Facebook, 1 was only on MySpace, and the last 2 were possibly on MySpace (they were more common names and had private profiles, so it was hard to tell for sure). Both of the 2 Facebook teens I asked "friended" me, and their profiles looked very active. In contrast, the MySpace profiles list the last activity date and most of the teens on the list hadn't used it in several months. If I was going to choose one site over the other to maintain for this library, it would definitely be Facebook.