It has already been a few weeks so she may have forgotten who you are or the first impression of you when you met. However, due to what you've provided I am assuming that she remembers you since she asked you to add her on fb.
Well, since FB deals with text only you can't convey body language or tone - that's a given. Disqualify her as the opener. Since it's been weeks you need to find a reason to even be on her page or you look like a stalker! Just make an innocent comment to get a response. For this example's sake let's just assume her name is Jess.
"Hey Jess.. where do I know you from again? .. if that's even your real name :O " << thats a shocked face btw haha
Just pretend that you forgot some details surrounding how you met. She won't get offended since it conveys the context as a joke with a blatant disqualifier. You WILL get away with it.
She will probably say something that at minimum resembles "we met at so-and-so's party"
say "hm. thats unfortunate"
and immediately following that post, say something like "hey btw, this is completely random but (change into a funny short story)" I find that funny short stories are the best since they are spontaneous and don't demand anything of the target. Through simple reciprocity - provided that your story was interesting to a certain extent she WILL respond.
Don't try a question opener - do it after her response to your story if need be, otherwise you have a natural conversation that can be embedded with attraction techniques aka PUA game automatically started.
She might you ask why its unfortunate but whether you respond to that initially doesnt matter since you have a second conversation stream already started.
If she presses for why its unfortunate, make a joke nonchalantly. "Oh, cuz you smell bad
"
Don't tease too much or you will lose her if she thinks you just tell jokes all the time - transfer into PUA game after she is responding to you.
CHAT