Those facebook improvements made there way back into the core.
Google might be able to move quicker if they allowed php. Facebook killed them in social. The type of products google can offer is limited by this decision.
Google+ did not fail for technical reasons. Everything I saw about it was technically better. It failed for social reasons, facebook already exists and has everyone on it.
It didn't failed for Social Reason, it failed because it was an inferior product for the mass. Facebook succeeded because its concept is simple enough for even your GrandMa and Dad to understand.
All of those did something different to facebook. None of them attempt to replace it. Googles problem is not that they can't move fast, they already move too fast. Every year they totally rebuild an IM platform and create a new music streaming service.
HHVM can't make its way entirely back to core, and is in fact completely separating from PHP entirely now[1]. If I recall correctly, while PHP7 implemented better op-caching, it has _not_ implemented JIT compilation yet.
This is true. But HHVM lit a fire under the php devs to get php7 out the door with significant performance improvements. (php6 never saw the light of day). Php has improved so much with the 7x releases.