I hadn't heard that Facebook had any major Postgres deployment. MySQL seems to form the core data storage for the appplication. Cassandra looks like something they will be using more and more of. They use their own Hive project to allow use of SQL to analyze data in Hadoop.

As for other big users of Postgres, Hi5 is a decent sized social net that uses Postgres. NTT uses postgres, and I think the coming simple replication and hot-standby features are based on code they developed for in-house use.