She won't add me?

Met a girl who also goes to my school at a party. We spent the whole night talking and got along pretty well, so I sent her a friend request on Facebook that night. Now several days later, she still hasn't added me. Knowing her, she's a huge Facebook-er who's always on every minute, and has over 900+ friends, but still she won't/hasn't added me. What's going on here? Has anyone else experienced this type of scenario? Thanks in advance everyone!