This is right. Source: worked on the FB MySQL team, though it was a long time ago. There is also massive, massive associated caching infra (memcached, Tao). There isn't as much hoopla about it but there are some public blog posts. I think it's even more interesting than the backing store.

The UDB tier used to use the InnoDB engine; nowadays it's MyRocks: https://research.facebook.com/publications/myrocks-lsm-tree-...