Answer by Thejo Kote:
May not be comparable to Facebook, but Skype and hi5.com use Postgres.
Stats from Skype deployment (http://www.slideshare.ne
t/adorepump/small-overvie w-of-skype-database-tools ):
- Over 100 database servers and 200 databases
- Largest table has several billions of records.
- Over 10000 transactions per second.
- Databases are used mainly for web store, billing and other centrally provided services.
- All accessed through stored procedures.