Thanks. Our complete stack is quite complicated, but a quick run down of what we're using: mysql, memcached, mongodb, gearman, nodejs, redis and puppet. (Node and redis are not in production currently.) The main language we use is php. I know, ugh, from all of you ruby and python guys, but good clean php is as good as any of the other language.

Yes, we have plans to expand the user demographic. Our product roadmap should help with this. We're just too constrained right now with engineering resources.

Wow, mysql, mongo, and redis all in the same stack sounds a little bit like you guys have fallen into the trap of chasing too many shiny new technologies :) (happens to me too)

MySQL is what we started with, as time has gone by, we've migrated more and more data to other data stores.

Redis and mongo solve two different problems for us. Redis is great for the social graph, which is how we've started experimenting with it, whereas mongo is best for just about everything else that doesn't hold the data in MySQL.