It's less a matter of scale to me and more a matter of facilities. Given the ability to scale PostgreSQL in places like Amazon RDS into utterly enormous system proportions, way less about performance to me. Is your data heterogeneous at a record-level? Do you store hierarchical data more than relational? Mongo might be a better choice with a schema-tracking system on top of it.

Saying that Mongo is the choice just for scaling is silly.

That said, Postgres is my go-to database as well, almost always, and I've never much needed anything different. Yet. Waiting to find something that it doesn't fit.