They haven’t monetized Messenger or WhatsApp much either. Maybe they can’t or it won’t ever be enough. Less than billions in profit won’t help much. Which does make them having won and are now going down seem more correct.
They "sort-of" did. The Open Compute Project (https://www.opencompute.org/) is an effort in which Facebook played a major role. The motivation, for them, is to break free of the shackles put on them by computing and networking equipment vendors (like Cisco). Facebook is big enough that they could design their own stuff, and try to make it a commodity which, as a side-effect, allows others to get involved and benefit from as well. It's hard to say how well this has worked but it was "an initiative".
Because exactly as you said: Facebook "won" the social network game several years ago: Everybody was in there. From there the only thing it could do to try to milk more profits was to bring it down.
Instead, we've got IRC or Usenet that, years and years after their "peak", they are still going well and good. Gone are the "ethernal septembers" and most of spam. And the protocol and networks (that matter for the people using it) are still in there.
You could also say that "they won" and there's no where to go but down now.
Nothing can grow forever and things that try are known, in the biological domain, as cancers or infestations. There's quite a few corporations that "won" their market, so they're primed for a decline-- if you can still even call it that after enough people made more money than god and retired.