They all do, through kernel patches, funding, open-source releases, talks and a lot more. Google especially has done a lot.
But that doesn’t change the fact that the operating system Facebook, Google or Android uses is an implementation detail of the main service they offer.
If Google or Facebook would have had to pay the full license fees for one of the major Unix distributions they would have simply never gotten off the ground in the first place. One of those 'uncomfortable truths' I guess that people simply don't want to acknowledge.
I’m not really sure what point you’re trying to make. For lack of a better word you seem salty about something but it’s really not clear exactly what. Do you want a 90's era “powered by Linux” image on the Google or Facebook homepage?
However the fact remains: Facebook’s use of PHP is an implementation detail, as is it’s use of Linux.
All these companies making untold billions off the back of FOSS could at least be kind enough to acknowledge that fact, rather than to bury it.