Any sane person dealing with sensitive, closed-source code would host their code themselves. Actually, host pretty much everything they can on their own network. Which is kind of a given if you're dealing with thousands of employees at your offices and billions of visitors on your website. (That or spinning up a s***ton of Amazon or Google cloud instances.) And this is one of GitHub's commercial offering, actually. GitHub Enterprise
lets you host your own instance of GitHub on your fancy company network with LDAP integration if you want and stuff for enterprises.
Or if you're just a small company, you just need a good VPS, install Gogs and everything you need that have an open source solution you can install yourself and you're ready to go, but of course, you kinda need someone who's kinda good with Linux. That's pretty much what we do at that place I work for, the only expenses we have is pretty much getting the employees paid and a $10/month VPS full of open-source software at our favourite hosting provider. Easy profits, y'all. Same on CodeWalrus, except we don't even pay our employees.
Speaking of Facebook, you'd be surprised, you say they're not really into coding, but I know the social network itself is a huge behemoth, I mean, they even wrote their own version of PHP in hopes it would work at least decently. Twice. Well, yeah, they're not really into hosting as much as Google or Amazon are, but it's still pretty scary to think of.