It takes a certain type of courage to live your life "out there". Some do it out of ego. Some do it to become shills for their latest activity or for others.
I have a Facebook account, but use it more to keep up with friends and share info we might all be interested in. I've screened out all of the folks who post pictures of their lunch, and I limit posts to things I think somebody might care to know or enjoy. It's no different than carrying on a conversation over a meal.
Who sees the stuff I post? Mostly, friends in real life, but I keep in mind that it's all out there, ready to be forwarded.