I am part of a group for meeting people in my local area. For a while I blindly accepted friend requests from people in the group, until it got to a point where I didn't really know half the people on my friends list, and it annoyed me because I was always scrolling through reams of stuff I didn't know about. Then one day I accepted a request and the person just kept posting massively racist stuff on their wall 
That was a turning point. So I decided I would only accept friend requests from people I know fairly well, have spoken to a fair few times, felt a connection or a friendship with if that makes sense.
Fine. So somebody sent me a friend request. I sent a note back saying (roughly) "Sorry nothing personal, but I only accept people I've got to know in real life. Hope to see you around some time :)"
She sent me one back saying "Oh, I thought I met you at X's house that time. Never mind, it must have been somebody else." Well, I did meet her at X's house. But speaking to somebody once does not equal getting to know somebody, in my opinion.
So then I sent back some long involved thing about being overwhelmed with a giant list and I think she was offended :(
DH reckons I should have been less apologetic and wordy in the first place and just said "Sorry I only have close friends and family on facebook".