Earlier quoted context omitted.
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.
However the fact remains: Facebook’s use of PHP is an implementation detail, as is it’s use of Linux.