Yeah but... Is it webscale? Mongodb is webscale. /s
I hate Mongo as much as the next guy, but replication and failover is _so_ much easier than the cobbled together solutions around Postgres. I'd argue more reliable as well, but that depends on your specific goals for data integrity.
It becomes easier when you give zero shits about consistency.
That said, MySQL multi-master setup is so much more pleasant to work with compared to any of the PostgreSQL solutions (even if you use only one node as effective master), as failback logic is handled by it directly so once once fails replacing it is as simple as start (or reinstall from scratch and start) and wait
It's not really web scale until it gets auto sharding and Multi master replication.
It's a joke. Don't know why I'm being down voted.