#1: If they’re not your friend, don’t pretend. Don’t accept friend requests unless you absolutely know who they are. There have been reported instances of private investigators, police officers, sex offenders and the like creating fake accounts to gain access to you online. If you do not know the person, then do not accept their friend request.
#2: Post only what you want made public. If you don't want your parents or co-workers seeing it, don't post it. Be cautious about the personal information that you post on any social media site. It may be fun to think that an old flame can contact you, but now scammers and thieves are clambering to access that personal information as well.