I use it mostly to store player-created levels and leaderboards which allows developers to efficiently + effortlessly include and filter by custom fields and data.

I also use it for some smaller tasks like storing filenames/hashes for distributing content across my 5 servers (doesn't really need to be mongo for this), and some configuration information for games using me.

I do it via MongoHQ, I did have it running and replicated across 2 of my servers but I'm just not familiar enough to keep it running and that bit me in the ass early on. With MongoHQ all I have to worry about is making sure my indexes are right.

Next up I'll be using it for some non-aggregated analytics stuff, which won't be as big as Facebook's requirements but will still be in the billions-a-month range.