"We had to think about how to build the architecture a little bit," Austin said. The approach they used, called architectural tiling, was designed to "build a system that scales to an arbitrary number of users. And we did that with SQL," Austin said. " We feel confident we can scale beyond 100 million users no problem."

Another big user of MySQL is Twitter.

Currently, Twitter has over 140 million active users, who issue about 400 million Twitter messages every day, all of which have to be stored, indexed and annotated with metadata. The company uses a modified version of MySQL 5.5 to handle this load. The company has six full time database administrators, to maintain "a few thousand database servers," said Jeremy Cole, the chief database architect for Twitter. The company also employs one full-time MySQL developer.

Cole spoke to why Twitter uses MySQL even when NoSQL databases would seemingly be better suited for such a heavy workload. "It comes down to a few key points," Cole said.

One is basic familiarity. "We have very extensive at-scale knowledge. We know how MySQL works internally. We know how to upgrade it, we know how to fix bugs and push out new releases," Cole said.