I've had a Facebook page for several months now. I use it mostly for personal communication - although some work friends are there as well.
It was great fun to set up. A friend recommended I do it - he was putting together a reunion of sorts. People who grew up in Dallas in the 70s and supported local music at this one particular bar. (Drinking age was 18 then - you could still be growing up and legally hang out at a bar.) It was called the "DJ's Class of 79 Reunion." This is as close as I will ever come to attending a high school reunion - so I got on board and set up a page. It was really fun for those first few days - as people I hadn't seen or talked to in 30 years were contacting me to be friends and I was talking to them all at once. It was very multi-dimensional and kind of surreal. (I'm a fan of Surrealism on Facebook).
I re-connected with some old friends and got some of my family and current friends on there as well. I would sit & watch TV and chat w/ my friend in Seattle while another friend in Austin uploaded pictures of his Cardoon plants. (a relative of the artichoke). It is very addictive. I'm not at the point where some of my friends are - updating their status many times a day with things like "I'm going to eat some empanadas" or "I'm going to work out now." I mean ... who cares?
I have friends that make things or are in bands - and that all keep me updated with pictures of their stuff or videos of their shows - as well as upcoming events. We all post pictures and commentary from our lives. It's a great way to stay in touch without actually seeing anybody. Although I went to the actual reunion this last weekend in Dallas and it was a blast. Still, I don't think I would be as enthusiastic about it if I actually had to see and talk to all those people all the time. It's staying in touch without actually touching.
I'm a fan of local museums and music venues - so they post regular updates with schedules and such. This is where I see it working within the Library system - with programs, new books, blogs, etc. (if the city would ever permit it). I get updates from THE DAILY SHOW. People recommend blogs and sites they like. That's how I discovered Texts From Last Night. Be warned, it is quite raunchy which also makes me see how hard it would be to control content if you were worried about that sort of thing.
Some of my FB friends gripe about the applications (Aps). They consider them an invasion of privacy - as they pull all your profile information and that of all your friends as well. So every time you send a "Punk Rock Single" or take a "What Movie Psycho Are You" quiz - you're invading everyone's privacy. I suppose if you are of a paranoid bent - that might be a problem. Personally - I don't have any real personal information in my profile - (DOB, Phone number, address etc.). My thinking is - if you want to stay private - why are you on a social networking site?