Actually, everyone in my family has an account in the household. My FIL doesn't but my MIL does, my 90-year-old grandfather doesn't, but his 90-year-old wife does, etc. I do have one friend who is still staying strong and holding out against Facebook...but except for the occasional Christmas card and email, I never hear from him, so that's kind of sad. FB would help me to feel better connected to him.
No one on my friend-list really uses it the way you describe, though (I wouldn't keep them around if they did, I think), and I personally find mamapedia to be a bigger time-consumer. My FB privacy settings and friend choices are pretty strict, so I haven't experienced any of the drama one of the other posters referenced, either.
I went to high school in two different states, college in two different countries, and have moved to a new city/state every 5 years since then. Facebook has helped me connect with old friends that I thought I had lost (my best friend from 8th grade who has lived in Poland since then recently found me!) as well as helping me maintain friendships with those I've left behind. I like being able to share videos of my kids with my mother, or see photos of my high-school friends' kids.
Having said that, though, I don't think you should get one just for the sake of having one. If you haven't had the occasion to really miss having it, why bother? It certainly DOES have an element of Big Corporate Brother that can be a little scary, and if your friends and family are all located nearby, I wouldn't think it would serve much purpose.