Mongo is specifically designed for horizontal scaling, so it makes it easy to establish replica sets and shards, and it includes an auto-balancer that will distribute your data between the shards. Pg (the database, not the guy!) can do much of this, but it's not quite as built-in.
Which isn't to say there aren't things that Pg is better for. Full text search comes to mind.
Which isn't to say there aren't things that Pg is better for. Full text search comes to mind.