It is nearly impossible to completely separate professional and personal contacts. What happens when a friend comes to work at your company? What happens when you and some coworkers start hanging out together a lot and become friends?
Instead of trying to maintain separate identities, I recommend having one identity and moderating what you post. First, as Brent pointed out, anything you post on the Internet is public, so remember that when considering those compromising photos from that party you only barely remember. Second, instead of separate identities, use filters -- security groups, circles, whatever your social network calls them, decide which of your friends/followers/contacts can see what content and only make the safe stuff visible to the world. If your social network doesn't support security groups, get one that does -- there's really no excuse not to have that feature these days.
Remember, though, that rule 1 still applies -- you've posted something to the Internet, albeit under lock, and a single weak link -- that is, one gossipy member of a security group, or one person you've had a falling-out with -- stands between your secret and a wider audience.