What do you do if a co-worker sends you a friend request on facebook that you want to reject?

I’ve recently started a new job and today I asked a co-worker of mine if I could send her a friend request (and I made it clear that it would be okay if she said no) but she was cool with it so I sent her a request, which she accepted. However, there’s really no else in the store I want to be fb friends with. I don’t even know if this woman is friends with anyone else in the store and no one saw me ask her, but I’m worried that one of the other co-workers may send me a request. I’d really prefer to be fb friends only with this girl.

This may change in the future, but for right now, I’d like a polite but firm way to tell a co-worker that I don’t want to be fb friends with them. I would just say that I don’t friend co-workers, but since I’ve already friended this girl, that excuse pretty much goes out the window.