It sounds like there is enough slack in the schedule that teams can decide they want to spend non-trivial amounts of time on these projects. It's surprising to hear that anyone, even the companies with big budgets, are able to hire enough people to do this without the projects getting an official seal of approval and budget. Even just taking the time to document, package, and publish is non-trivial.
It seems like the vast majority of companies are not far enough out in front of their production issues and requests from the business side that engineers could do this sort of thing. So I guess it's impressive that Facebook (and probably Google) are in that position.
There is a degree to which scale makes it necessary to develop these sorts of projects. Losing 1% of productivity in a engineering group of 10 people might not be a big thing, but at 1000 people that's 10 full-time people worth of productivity you're losing. Dedicating 1-10 people for a few months or a year (or even two years) to remove that 1% productivity loss is clearly worth it.
Documentation costs productivity to write, but when there are many people who would be made more productive from it, it makes sense to do it.
I think "slack" is the right way to think about it. It isn't a free-for-all - the business still needs to run - but there's enough space to explore, to rewrite, to document, to polish, and so forth. That's where the magic happens - the unplanned and the unexpected things around that which you thought you were going to do.
It seems like the vast majority of companies are not far enough out in front of their production issues and requests from the business side that engineers could do this sort of thing. So I guess it's impressive that Facebook (and probably Google) are in that position.