Yes but I wonder how much it will really help. Is the bottleneck for Facebook-like sites really in the bytecode dispatching as opposed to e.g. string/regexp operations or database calls?
It seems sensible to assume it helps them for something otherwise why bother developing and releasing it? I also don't think it's a matter of bottleneck - if you make the front-end php run faster you need fewer servers, fewer servers/user, more money. This has come up in discussions about google as well - there are some scales at which even comparatively modest improvements in efficiency count. It probably won't make a world of difference to My Cousin's Personal Basement URL Shortener.
>It seems sensible to assume it helps them for something otherwise why bother developing and releasing it?
Well surely the proof is in the eating - that is, if they've implemented it then it clearly works, they wouldn't be stupid enough to implement it if it's going to cost them money.
To answer your [rhetorical] question: The cost for this has been one developer for a year. That's not much for a speculative shot at increased server efficiencies that could save them big money in the server farm. Once they've developed it, even if it didn't produce the gains they needed then releasing it is sensible as it gains them some PR amongst the OS community and may get more fixes from that same community improving the result (lower costs) for FB.
Sorry I read somewhere else that it was a single dev and that the gains were expected to be 5x improvement in speed for the same processing power. Can't recall where.
As far as I can tell Facebook use memcached so extensively that practically everything is served out of it, so their bottleneck could well be the PHP application code as opposed to the data fetching. In any case, when you're serving potentially millions of page views a second even a small optimisation can save you from buying a ton of extra hardware.
JIT native code compilers do make quite a bit of difference over byte code execution is some situations. E.g. loops over many elements (say loops that concatenate large HTML fragments, ok the string operation are already native, but other bits of the loop will not be (e.g. array access)).
So, I agree that the main 'bottleneck' in most PHP apps is probably not-PHP itself, there is still scope for improvement.