"Everyone" meaning a few people who care more about the social network's design than communicating with the people who use it? I find it very hard to believe that there's going to be some kind of mass de-platforming event that leaves Facebook a ghost town. Maybe because of changes to political content policies, but definitely not because of UI changes.
Snapchat has gone through a number of major, controversial redesigns. It's definitely not going anywhere (at least among its target demographic). Facebook is far more entrenched than Snapchat; its target audience is less quick to switch platforms as well.
It begins with those who have the know-how to do it today, and they are bringing their friends and family, and the acceleration is exponential.
Look at Digg: two-fold cause of 1) UI changes, and 2) user perceptions of centrally controlled, biased, and influenced information flow.
Digg was thought to have unbreakable critical mass.
Now, Reddit and Facebook are both going down the same road, with user-hostile UI and opaque information sourcing.
People aren't dumb, and they've already noticed and made up their mind about it. Now they're just looking for somewhere to jump.
I recently interviewed two kids no older than 20 for a user study. Not techies. One of them doesn't use Snapchat for privacy reasons. Other one thought about it and said, "I don't know why I use it, because I know that it's not really private." I didn't even prompt them with any questions about privacy or control, I was just doing a cursory "what apps or social networks do you use?"
More and more people are saying, "I know it's crap, but I'm on there because everyone else is using it." It means they're already looking for another place to go.
Snapchat has gone through a number of major, controversial redesigns. It's definitely not going anywhere (at least among its target demographic). Facebook is far more entrenched than Snapchat; its target audience is less quick to switch platforms as well.