I think it's hard to say what would happen, but personally I think you can't compare Facebook and SteemIt. They're are nothing alike.
Maybe the major part of the people who signed up for SteemIt during the last two months will switch to Facebook again, but I don't think that the people who came here with the right intentions would leave Steem and hurry back to FB. I know I wouldn't. (Although I think I'd use FB to earn some easy money on the side - creating a post on FB doesn't take me hours to do, creating a post on SteemIt does - lol)

I think a Facebook Blockchain token would be very beneficial for the crypto world in general. It would surely speed up mass-adoption.