From a very old article still quite relevant:
Facebook’s "Domas Mituzas" shared some stats that illustrate the importance of its MySQL user database: MySQL handles pretty much every user interaction: likes, shares, status updates, alerts, requests, etc.
At 2010/11, Facebook had 800 million users; 500 million of them visit the site daily. 350 million mobile users are constantly pushing and pulling status updates. 7 million applications and web sites are integrated into the Facebook platform.
User data sets are made even larger by taking into account both scope and time And, as Mituzas pointed out, everything on Facebook is social, so every action has a ripple effect that spreads beyond that specific user. “It’s not just about me accessing some object,” he said. “It’s also about analyzing and ranking through that include all my friends’ activities.” The result (although Mituzas noted these numbers are somewhat outdated) is 60 million queries per second, and nearly 4 million row changes per second.
At Facebook they used MySQL to manage many petabytes of data, along with the InnoDB storage engine that serves social activities such as likes, comments, and shares. While InnoDB provided them great performance and reliability for a variety of workloads, it seemed to fail on space and write amplification when used with flash storage.
As a result they developed, fully opensource solution MyRocks: A RocksDB storage engine with MySQL, an embeddable, persistent key-value store. It's leveraging stability of MySQL and taking advantage of persistent Key-Value Store for Flash and RAM Storage(RocksDB).