week.. you should ask her out.. If you know her and she's not a friend of a friend call her.. When guys I don't really know try to flirt with me on FB I don't really care.. But I'm always nice and say thank you.. But FB isn't a dating Site and that stuff makes me super uncomfortable
Asking her out now is too straight forward and makes you seem like a total creeper. Just get to know her. Let her talk about herself and find out what she likes. From all that info you'll know where to ask her out to.
Say you're welcome. Then find common ground to talk about. If she's talking, listen and if she asks for your opinion, be honest (if it's a negative responce, say it in the most gentle way possible). If she's interested too, she won't be nervous to continue talking to you. Best of luck!
Just start a conversation with her so you don't seem just like some guy calling her pretty. And next time, you should either say she's pretty on her fb picture so you don't have to say anything afterward, or when you see her in person, try to notice something different about her or if she looks especially good that day. Then be like, "Did you do something different with your hair? It looks good." and things like that to show her you notice those things, even if you really don't. Don't say things like, "You look really pretty TODAY" though. Some girls might not think about it badly, but I do and I know other girls that do too because it sounds like you're saying she looks good that day, but every other days she looks average.