I would say that it isn't configured to scale like Mongo out of the box...but that doesn't mean it can't.
You can go outside of Postgres core to get multi-master solutions with easy sharding and clustering...with the open sourcing of CitusDB and 2nd Quadrant's pglogical and BDR extensions there are options out there.
You can also roll your own (if you really want)...and it is relatively approachable to do so using built-in features like partitioning.
And, of course, with the 10 Beta it would seem that logical replication is being brought into core which sets the foundation for future replication features such as BDR to also be brought into core.
I would also point out that since Mongo's BI connector fiasco, it would seem more and more Mongo users are finding more reasons to just use Postgres (where relational interfaces are desirable for BI): https://www.linkedin.com/pulse/mongodb-32-now-powered-postgr...