I confess, I'm a lurker. I used to be very active on Facebook, but several things turned me off from being so active. There were some legitimate safety/privacy concerns that popped up about three years ago. There was no good way to protect my family just by tweaking privacy settings due to the interconnectedness of everyone/everything on there. That was probably the biggest issue. Then there was the inevitable drama, and after a completely innocent post on my part turned into a bashing contest between a good friend and a cousin of mine, it took a lot of the fun out of posting things. I don't like drama, and I don't like being caught in the middle of a feud between a friend I love and family I tolerate :D. And then came the point where everything I thought about posting was so cliche and annoying even to me, I decided to self-censor, and ever since then, I just don't do much on there.
However, I enjoy reading what everyone else is doing, especially since we've moved and can't keep up with people quite as well. While I don't do a lot of public stuff on there, I frequently exchange private messages with friends on there, and since a lot of people now use that instead of regular email, that's just what works for me.
As far as the comment about letting people have their privacy by not lurking, if someone has invited me to be their friend or has accepted my friend invitation, I don't consider it an invasion of their privacy to catch up with them even if I'm not commenting on every single thing they have to say. It's all part and parcel of the Facebook experience that anyone you've allowed in could be reading what you have to say.