I use Facebook to keep up to date with people who I want in my life and yet for one reason or another (money, distance, etc) I can't meet up with or speak to on the phone. Plus it's useful to see what others are doing and reply to them, or just be nosey. BUT I am very careful and conscientious with who I have on there as family, old friends, schoolfriends and even ex girlfriends are on there so every addition is considered with plenty of thought.
I'm on my own quite a bit of the week as I'm single, live on my own and currently work from home, so sites like the above, plus Forums like Tom's are vital to me...and good fun too!