Well, yes and no.

Everybody knows that relational databases don't scale because they use JOINs and write to disk.

Also, relational databases weren't built for web scale. MongoDB handles web scale. You turn it on and it scales right up.

And before you knock shards, shards are the secret ingredient in the web scale sauce. They just work.

Furthermore, relational databases have impetus mismatch, and Postgresql is slow as a dog. MongoDB will run circles around Postgresql because MongoDB is web scale.