I see the point people are trying to make about Facebook killing your "mystery" and all that, but that is completely outweighed by the good it can do if used properly. I just got out of a LTR, and the majority of people I hung out with were also in relationships at the time. I lost touch with so many people. While posting some things gives more info to people, its also gotten me plans to go out with new people that I ended up having great nights with, including plenty of opportunities for pickups I wouldn't have had with my "relatioonship friends".
I also just had a girl I hadn't talk to in a while message me on Facebook because she saw I was single. That turned into the least amount of work possible to get her in my bed.
No one said you need to go add the girl you just met at a bar that night to Facebook. There is just no point to that. But considering the purpose of Facebook is to be friends with, well... friends.... I don't see why that "mystery" is so important.
One thing I will say, I miss the original Facebook. I was a freshman in college when it started up (for college students only) and that thing was a gold mine when it came to social life, school work, etc. I try to keep my activities on there limited to the original purpose of FB (comments, messages, events, pictures....with FRIENDS)