I dont use any, I do look on them occasionally to see if any people I used to know have won the lotto and to be nosey. To be honest most peeps I know on these social sites I spent years trying to get away from.
That and the fact that theirs enough personal data floating about the www about everyone, no need to add to it. It's a criminals heaven when you can find out so much info about someone, all offered up for free!! Or am I being paranoid.