My personal opinion on it is that this is in fact a very efficient way of doing things at Facebook. Facebook probably doesn't hire more than a couple people working on HHVM. Let's say their salaries individually are around $100,000 a year. That's cheap for them versus the value they're getting out of it.
Facebook probably has millions of lines of PHP code, PHP code proven to work. This isn't just Facebook.com, but internal websites, their bug tracker software (that is open source), analytical software for PHP, etc. etc. Rewriting all that would be _expensive_.
Running it on normal Zend PHP, based on the benchmarks they get on HipHop, requires _way_ more hardware at the scale of Facebook, so again, very expensive.
PHP developers are abundant, PHP is a relatively easy language to teach, resources are everywhere. Due to their abundance, PHP developers (even at Facebook quality) are probably less expensive than other languages. Easier hiring and lower cost developers, cheaper.
Facebook probably has millions of lines of PHP code, PHP code proven to work. This isn't just Facebook.com, but internal websites, their bug tracker software (that is open source), analytical software for PHP, etc. etc. Rewriting all that would be _expensive_.
Running it on normal Zend PHP, based on the benchmarks they get on HipHop, requires _way_ more hardware at the scale of Facebook, so again, very expensive.
PHP developers are abundant, PHP is a relatively easy language to teach, resources are everywhere. Due to their abundance, PHP developers (even at Facebook quality) are probably less expensive than other languages. Easier hiring and lower cost developers, cheaper.