It’s certainly not used for any mission critical apps. Facebook’s stack is pretty well known. They’ve been using sharded MySQL for a while now. Instagram started on PostgreSQL but I believe has switched to Cassandra.
Thanks for the clarification.
As an example: would you consider the backend software stack that manages physical access to the campus 'mission critical'?
Mission critical - essential for operating Facebook.com
Is is a company wide system? Its all fun and games until the cooling in a dc starts to have issues and the facilities team can't gain access because....mongo... ?
Disclosure: I worked at Facebook, but not in that department, or anywhere near any MongoDB.
In much the same way I wouldn't say that my site is powered by Microsoft Excel; but you can be sure Microsoft Excel is used in my company.