Ubiquity Edgerouters' management interface has a MongoDB backend and I have one on my desk at work, but I would not consider that I run "MongoDB" to provide services for my customers.
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.
I think they mean for first party backend tools. For example at my job I would say we use react in production - not on the frontend, but only for an internal warehouse application interface, nobody outside can use it but it's vital to our business, and we're the ones who built the application.
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... ?
Sure, that's mission critical (ish), but it's not really what people think of when they think of Facebook. It's not millions of posts per second, ultra scalable whatever.
Disclosure: I worked at Facebook, but not in that department, or anywhere near any MongoDB.
