"So face" is a play on the words "so fake" in relation to Facebook comments, statuses, and well wishes. It points out the pretentious, contrived, and fake information many Facebook users willfully exchange with Friends, or make public from their Profiles. Things like status updates on how they're feeling at any given moment; or that they're so busy or late yet have the time to update their statuses declaring this; or Happy Birthday exchanges between people who only get in touch once a year during their date of birth. There are many other instances of Facebook users feigning interest or positive support.
(You would use it just like you would use the words, "so fake,' but in this case, specifically to Facebook related actions). "She is so face!"; or "You are so face"; or "That is so face!"; or "Look what this person I haven't spoken to in years wrote on my wall, she is so face!"