It's too early to tell, but because they have such a wide reach and own 1/2 of mobile, if they push it hard and it's a clean enough solution it may just work as a relatively 'privacy first' orientation.
But then websites don't get their FB pixel data, can't retarget and build ad audiences. FB also does some heavy lifting in terms of managing forum comments, which can be useful in some specific situations.
As long as people are willing to click in the FB/G sign in buttons, I think they'll be there to use.
I suggest the biggest opportunity for change may come in the form of regulation and legislation.
