It was released post-buyout, so that makes no sense.

Well, may not exactly big but Quora is using Tornado in production.

Hipmunk is using it.

I am not big or even production but unscathed.com is built on it. Development has been active and the project is well maintained and judging by the group it is gaining momentum.

Bit.ly appears to be using it. They even have a module they released for using tornado and mongodb. https://github.com/bitly/asyncmongo

We're using it in production at Braintree.

bit.ly uses it in production

We are using it in production at www.mixcloud.com to serve any long polled connections

We use it as well at Summify