In my case it's all but replaced e-mail for communication with family and friends, and as someone with family and friends spread all over the world it is a great way of keeping in touch as well as re-establishing contact. For example, I like the way I can put up family photos and people can see them and comment on them if they like, but without the same obligation implied by an e-mail message. Same with exchanging information, opinions, ideas.
My e-mail is now almost entirely work-related and almost all my social communication is via Facebook. But it's really what you make of it. If you're concerned about privacy keep the personal details to the minimum. If you dislike all the trivia don't take part in it. If you don't want to inflate your friends list with people who aren't really your friends, don't send them or accept their friend requests. I got the hang of this as I went along.