Not a migration to NoSQL but checkout these exciting presentations and blogposts on Uber's migration, they started out on MySQL then migrated to Postgres [0]

Then as they blitzcaled they famously migrated again from Postgres back to MySQL 3 years later. [1] [2]

Seems that MySQL is good enough at Uber's scale.

[0] https://www.yumpu.com/en/document/view/53683323/migrating-ub...

[1] https://eng.uber.com/postgres-to-mysql-migration/

[2] https://use-the-index-luke.com/blog/2016-07-29/on-ubers-choi...