So they dropped some rarely used functionality from PHP,... like serialization. Excuse me, "rarely used"?? Drupal strongly depends on serialization, to store data structures in the database (or, as they call it, "the cache"). You cannot effectively run Drupal without serialization. So "The likes of Drupal, MediaWiki, and WordPress are now using HipHop." sounds like it cannot be true.
edit: https://github.com/facebook/hiphop-php/wiki/Unimplemented-Fu...
That's a list of unimplemented functions - unserialize_callback_func is unimplemented, but that's it wrt serialization functions.