Nice find. However, I don't see where it says anything about MongoDB failing, though it is mentioned as a NoSQL database. Also:

Another customer, Electronic Arts, uses Basho infrastructure for a system that supports 7 million daily users of Warhammer Online on Facebook, saving each player’s status every half-minute, Falco said.

That's an impressive number of daily users, but it's just saving a status every half-minute? I don't see why MongoDB couldn't handle that just fine, assuming enough nodes (auto-sharding is basically working now right?).