The advantage of PHPNG is that it has a dedicated development team who will not go away when Facebook goes the way of Myspace. And, yes, I know it is open source and development can continue. My point still stands.
I once thought that Facebook would 'go the way of Myspace' too. But then I attended a USENIX LISA conference and I sat in on some of the Facebook presentations. Initially, I was dismissive of them (I'm too old to be in the social movement), but after seeing the advancements that Facebook are making with Open Compute and looking over some of their source code, I was very impressed. They had the best presentations out of everyone. It was all very surprising to me. And last but not least, the caliber and reputation of their technical staff is as good or better than the other big players. I came away with a much deeper respect for them.
If PHPNG dies they simply could elect a different environment to be the official one. Just because PHPNG is "the future" right now doesn't mean the fate of the language is tied to it forever.
Facebooks' ties with the NSA spying ring is what makes me think that. Oh, that and showing users their dead relatives pictures surrounded by festive themes around christmas time.
EDIT: I didn't think I would need to mention the obvious reason nhtechie there mentioned. That was my whole point after all, for the ones who understood it before downvoting, but whatever.
You're being downvoted because it's extremely foolish to think that Facebook will collapse due to the dead people on it. My theory is that people don't even know where to start on a rebuttal for such an argument given how absurd it is, so instead, they downvote.