I have experience using both MongoDB and PostgreSQL. While pretty much spoken here is true, there is one more scalability aspect. When a fast moving team builds its service, it tends to not care about scalability. And in PostgreSQL there are much much more features that prevent future scalability. It's so easy to use them when your DB cluster is young and small. It's so easy to wire them up into the service's DNA.

In MongoDB the situation is different. You have to deal with the bare minimum of a database. But in return your data design has much higher horizontal scalability survivability.

In the initial phase of your startup, choose MongoDB. It's easier to start and evolve in earlier stages. And later on, if you feel the need and have resources to scale PostgreSQL, move your data there.