The other business interest is... as it gets more adopted, more people will know the ins and outs, and the talent pool for them to pick from will grow.

More other businesses using it also makes it harder for people to make the argument against it as "it's just a Facebook thing"

Yes. It's probably hard to hire developers if they must program in an obscure internal-only dialect of PHP (Hack).

That comment got downvoted to -1! I'm not saying Hack is obscure and used only by Facebook - but it would have been had Facebook not open-sourced HHVM and tried to promote its use.

It's not that obscure. Anyone with an ounce of experience in PHP can start writing solid Hack on day one. And that would still be true even if Hack were never seen in the wild.

Unless you think it's hard changing the open tag from <?php to <?hh :p

I'd be surprised if anything-PHP is a bonus to hiring. Most likely people work on it despite of PHP, not because of it.

No, many people work on it because of PHP - because of the speed and flexibility of dev with it.

I've done PHP, some Ruby, some .NET, Java and more - coming back to PHP is often fun and a breath of fresh air. Nothing's perfect.