How do I say no to someone who wants to be my roommate?

It's not that I don't like her or I don't want to be her friend, but I don't know her that well. I've never met her and all we've done is message each other on Facebook. I want to decline politely because she seems like a nice girl who I would want to be friends with, but I don't want to room with her when I don't know her.