My personal observations are that a good number of people have felt 'fatigued' by Facebook for a very long time, but were also unsure of how best to extricate themselves without incurring a social penalty.
But now there's an impetus that most people can understand. I'm not sure about how many people will move away or how quickly it'll happen, but the network effects Facebook capitalized on can also work in reverse: if you have just one or two very vocal privacy proponents in a friend circle pushing to get off the platform. One group I'm in recently migrated to Telegram for this very reason.