Horizontal scaling is key. If we had more faith in auto-sharding in MongoDB 1.6, that may have worked (however horizontal scaling wasn't our only issue with MongoDB).
With PostgreSQL we manually partition the data (luckily we have a really nice key to partition on).
PostgreSQL 9's streaming replication has been adequate so far. We've only been using it in production for a few months and recently moved off of AWS* which drastically reduced our error rates overall.
Totally off-topic here, but have you noticed that the body text size of your company's blog stays really small even when trying to zoom in? I've tried zooming in using Safari and Chrome, and with both of them the text stays so small that it's difficult to read.
With PostgreSQL we manually partition the data (luckily we have a really nice key to partition on).
PostgreSQL 9's streaming replication has been adequate so far. We've only been using it in production for a few months and recently moved off of AWS* which drastically reduced our error rates overall.
* http://urbanairship.com/blog/2011/06/01/hybrid-cloud-were-up...