Skype uses PostgreSQL as their backend database

Skype scales PostgreSQL to handle billion of users

Skype's large PostgreSQL installation with primary focus on high availability, security and performance

" Their approach is to use a traditional stored procedure interface for accessing data and on top of that layer proxy servers which hash SQL requests to a set of database servers that actually carry out queries. The result is a horizontally partitioned system that they think will scale to handle 1 billion users."