This is one of the reasons that I personally dislike Facebook for sarging.

Generally speaking the women aren't exactly in the right frame of mind, and moving them towards that frame through an emotional response is difficult without any 'real life' interaction. A lot of women that I know will also instantly assume that you are a creep if you randomly initiate on Facebook.

That being said, if you're intent on continuing, it sounds like you need to build some more comfort with this girl. Keep the conversation cheeky, jovial, and covertly sexual, whilst letting on a bit more of your own personality.

Try to move into a topic that allows you to DHV.

EDIT: With regard to asking for her number, you've already moved on that front once. Perhaps chat for a while longer and then drop your own.