I started my foray into the world of social networking a couple of years ago with a MySpace account - like most things in my life, it managed to hold my interest for about 2 months and then dropped off my radar. I also found that most people were simply adding others to get their number of "Friends" (and I use that term very loosely) up. Something about it seems to fill that need most humans have to be "The Most Popular" - it just reminded me about bad days in highschool - something most people can probably relate to.
What also annoys me about MySpace is the amount of time it takes to load the image-laden, often badly designed pages - all the while being blasted by someone's idea of "good" (and I use that term loosely as well) music choice from their mini-ap song player.
The journey to Facebook kept my hooked, once again for around 2 months - every now and then a flurry of activity will occur with my "Friends" and my interest will be peaked but that invariably goes away and is often replaced by a comfy chair and a good book. I will say this for Facebook though - it was a great way to get in touch with old friends (of the real variety).
Possibilities for your library to use social networking for client interaction, marketing and networking opportunities - I started a Young Adult group last year titled "YAAH!" (Young.Adults.Arana.Hills) - during the first meeting we discussed how we'd like things to work - one of the questions being "Would you like to create a MySpace or Facebook page for use by the group?" - the answer was a resounding "NO". The group is a diverse range of teens aged 12-16 years and yet only around 50% of them even had a social networking account. Interesting...
