If Facebook were de-blocked today, you might very well see a mass migration. All my students (Chinese high schoolers and college transfers) going abroad immediately send me friend requests as soon as they get out from behind the Great Firewall.
Couldn't that be because once abroad, everyone they interact with are on Facebook?
Taiwan and Hong Kong have. If Facebook wasn't blocked in China, you would assume that trend would have continued.
And possibly Russia? Can anyone comment on how VKontakte is doing?
VK is the social network in Russia and probably in whole ex-USSR.
Censorship states will probably buck the trend due to the politics of migrating.