What are the biggest negatives of using social network sites like Facebook etc?

[cemistois]

yes, you can make your profile private, and depending your settings, you can make your age/and or location visible to someone that's not in your friend's list. you can make albums that are visible to only some of your friends, you can make your friends not being able to leave you comments on your wall etc.
however, if you know a little about looking up people (on spokeo or peekyou), your default picture WILL pop out. so, no matter how private your profile is, your info can and will pop out on other search engines.