Well I want to make sure that nobody assumes that just because I am not aware of any major website using ZF2, that its necessarily a fact. Quite likely nobody in the ZF2 community heard that both opensky.com and exercise.com launched on Symfony2 2 years ago.
well for Symfony2 there were two major sites running on Symfony2 pre alpha versions (2 years ago) .. and several others more in alpha. Obviously these teams had to spend a fair bit if time keeping up with all the BC breaks before the stable release, but it helped a lot in ensuring that Symfony2's stable release was ready for the real world.