Not the OP but also having worked on multi-million user apps, off the top of my head: zero downtime deployments, small failure domains (if you make a bad update to a service, only related functionality suffers, the rest keeps working), frequent small deployments (like, several times a day), easy and quick integration testing (cause you only have to test the functionality of one service, not the whole system), easier debugging, cause if one functionality isn't working, it's easy to analyze only the logs of the service responsible for that functionality (and optionally move to others later, having identified some part of the cause). Also, every microservice is a new clean slate, you can quickly learn from your mistakes and try out new approaches. (Not diametrically different, but you have the ability to iterate more)
Edit: another one, is that if you keep the microservices actually small and well described by an API, you can easily, quickly and safely heavily refactor/rewrite old services.
