First thing I thought was that this was a WG you'd seen previously, and you'd contacted her on your personal phone, linked to your FB account.
But it seems like she's just randomly found you. In which case it's as dodgy as fuck.
I would say there's a 1% chance of her being legit, ie she just chose you off FB out of all the other thousands, to have a hot affair with her.
As others have said, are you partnered, and could this be her trying to catch you out?
Also as others have alluded to, I don't know what your FB privacy settings are but is there a chance she already knows who your friends and family are? In which case this is 99% a scam, and she (if indeed it is a she, and not some Russian cunt sitting in a St. Petersburg Troll Farm, or maybe in Nigeria) will pretty soon start demanding blackmail payments.
Before you go any further, assess what she could already possibly know about you, and assume she will use it against you.
If she can't possibly know who your friends and family are from FB, and you're not partnered, and you have nothing in particular to lose, then maybe call her bluff and arrange a meet.
But it could still be a scam, she might know you from somewhere else anyway? Still sounds dodgy as fuck
.