To clarify, since you worded this a bit ambiguously: MySQL is the backing store for the graph database, which is the primary Facebook product database tier. But

To clarify, since you worded this a bit ambiguously: MySQL is the backing store for the graph database, which is the primary Facebook product database tier. But separate from that, MySQL is also used for a ton of other purposes at Facebook. Or at least it was as of a few years ago; AFAIK this is still the case. For example the internal MySQL DBaaS tier had hundreds of distinct use-cases throughout the company.