What we built is a Facebook connected game called Hatchlings Match. It's a spin-off game that reuses the art from our original game, Hatchlings, but has completely different gameplay.
Building on Firebase was great; it was also the first project we've built on node. The combination of those two things increased our development cycle tremendously.
So far the nicest thing about launching on Firebase has been that we haven't had to worry about scaling. We're adding over 800 users per hour right now (and Firebase is showing over 350 concurrent players currently online and connected) and that's all running off of a single (small) EC2 instance that's not doing much more than serving static files.
One other thing we're particularly proud of is that it (knock on wood) works completely cross platform and cross browser (tested so far on Chrome, Safari, Firefox, IE, iOS, and Android). [A friend has informed me that touch events aren't working on his MS Surface device -- trying to get ahold of one of those to test with at the moment.]
I'd be interested in any tips you all have for improvement (especially the onboarding process and the first level's tutorial). Or any ideas you'd have to help us make it more social.
Here's the link: https://www.facebook.com/appcenter/hatchlingsmatch