Our app does a lot of writing. We were on Mongo 1.8 which has a global write lock. So concurrency was a big problem under load. We also have a lot of users with networks of 200k+ people – not even including followers. This meant that we had to have arrays in the User collection with 200k+ IDs to perform a lot of our queries. The RAM needed for indexes wasn't really worth the cost and performance started to become a big concern. In the end, our data makes more sense in SQL with a cache layer (Redis in our case) rather than NoSQL.

Our experience thus far in Mongo vs Postgres: go with what you know. We probably could have made Mongo work for us (especially Mongo 2.0+), but it was easier for us to work out the performance issues with Postgres. Plus, things made more sense in SQL.

If you're using Mongo and need a cloud provider, I can recommend the people at MongoLab. They really went above and beyond helping us get up and running and are really cool people.