RocksDB is an awesome building block for building your own database. Now usually you don't need that. But if you have a ton of traffic, moving 1 abstraction level lower gives you a lot of room for optimization. Stream's in-house db is for instance using RocksDB. https://stackshare.io/stream/stream-and-go-news-feeds-for-ov...

Mixpanel, DataDog, Linkedin and Pinterest also created their own DB tech. Linkedin and Pinterest use RocksDB. Instagram lately started doing something similar by using RocksDB as the storage layer for their Cassandra cluster.

They are often used as the base storage layer for more complex things.

CockroachDB has RocksDB at the bottom: https://www.cockroachlabs.com/docs/stable/architecture/stora...

Etcd uses a fork of BoltDB: https://github.com/etcd-io/bbolt

Indeed uses an LSM tree based key value store for its data about job postings: https://engineering.indeedblog.com/blog/2013/10/serving-over...