MongoDB is at a scale community-wise where they are growing by leaps and bounds as MongoDB ends up in many places where MySQL used to rule and for many newer applications where the relational and transaction benefits of MySQL are unnecessary. You can find mature MongoDB bindings for everything and now assume that they are mature for pretty much every community you care about. When people come to programming for the first time, many are likely to now learn MongoDB as their first DB and not MySQL. With all this in mind, MongoDB is now basically a large snowball picking up lots of small wins everywhere because of ubiquity and gaining mass. MongoDB is the second major open source DB win since MySQL.

Given conformity, social proof, product awareness, large enough community to bet your non-tech business on it, do you doubt that it can't grow a bunch over the next 5+ years? It might be dead to you, but it's the next big thing to the rest of the world who are not as enlightened as we.