Japan has been known as a country where a shiny new technologies such as weird USB gadgets, cellphone TV/wallets or RFID devices are introduced first and adapted in a really early stage. Plack is not an exception.
Yesterday at the DeNA's technology seminar two of the biggest social networking sites in Japan, mixi and mobage-town (both articles on TechCrunch) revealed and explained that they are running their OpenSocial providers on Plack, using Starman and Twiggy (*1).
Also the mega portal livedoor is experimenting Plack stuff on their popular RSS/Atom feed reader livedoor reader API, combined with Coro, AnyEvent and nginx. I'm pretty sure Kyoto-based blogging and digg-like social news site Hatena also is testing Plack and hopefully we can get some news on that in upcoming YAPC in Tokyo.
Think Facebook, MySpace, Digg and Google Reader all use Perl, Plack and deploy their apps on all of those PSGI web servers. That's what's happening there!
Oh just in case you're curious: livedoor uses Sledge, the framework I developed 7 years ago while mixi, mobage-town and Hatena use their own in-house web frameworks rather than the ones on CPAN like Catalyst.
 Technically they're deploying the old Plack::Server::* versions on production but they consider switching to the new versions soon.