Eboracum as a peer-acclaimed connoisseur of women I'd suggest you don't take any stabs at what her intentions are, and just take your time.
There's a thousand possibilities as to why she wants to take things to Facebook first and quite frankly we don't have a hope of guessing them from this side of the screen.
Be patient young son. You're looking for a relationship for the long term so you better be ready for a chase that lasts longer too.