Probably just wants to be friends you know, no need to be an ass hole but you can be friends with people that are different? Don't you feel remotely sorry for her? Like if she is all there in the head then honestly there isn't any reason that you can't be friends with her? But you said Hi, and proceeded to add her on facebook. With this in mind she might have considered that you could be interested in her in a 'Boyfriend/Girlfriend' kind of way; to her anyway. Therefore, maybe you shouldn't talk to people that you don't want to be friends with? But as I said, ain't nothing wrong with being friends with a weird/not the best looking girl, or the most popular girl; but then again most people in school are judgemental fucks and will ask you what you're doing talking to her but you know fuck them people.