> Which language would have been a good technical decision in 2002-2003? It must be easy to onboard people who don't know it onto. It needs to be easy to operate, and not be costly to deploy on the tens, hundreds, and then thousands of servers necessary.

You mean, like the JVM? 2003 wasn't the pliocene epoch, we had a working JVM. If you remember back to the last bubble, in the 90s, we were shipping "easy to operate, not costly to deploy" software on Java post-1998. Java 1.4 was released in 2002, and Java 1.5 -- what most people would say is modern Java -- was only 2004.

Scala 2.0 was released in 2006, Clojure in 2007 -- that's 8 and 7 years ago, respectively.

You really don't think there were alternatives during that long period?

> What was bad about the decision to keep the reasonably well-performing and reasonably suited-to-purpose PHP code for front-end code, and peel off suitable tasks into services like the feed, typeahead, messages, and so forth into languages like C++, Java, and so forth.

In 2004? Nothing. In 2005-2008? Things should have been reassessed, especially before building out a millstone of an engineering team around PHP. Instead, Facebook doubled-down on an actively bad language with HPHP, and the results were hilarious:

   HPHPc required a very different push process,
   requiring a bigger than 1 GB binary to be compiled
   and distributed to many machines in short order.

So then in 2010, Facebook decides to embark on HHVM, and now four years later, we can run one of the most correctness-hostile programming languages around, quickly, with optional static typing.

That's a span of 6 years, and at the end of it, Facebook has functionality they could have gotten for free in 2003. On top of that, the intervening years allowed the PHP mess to become only more entrenched -- who on earth do you think the engineers are that accept a job writing PHP, for Facebook or otherwise?

If I had to hazard a guess, I'd guess that HHVM exists because of a large amount of political inertia in the organization that has everything to lose by PHP being eliminated entirely, and the lack of a strong hand by upper management.

I'd guess that lack of a strong hand by upper management came in no small part from hiring straight-out-of-college graduate Adam D'Angelo -- who had literally zero experience -- to serve as CTO from 2006-2008.

By the time FriendFeed was acquired and Bret Taylor along with it (2009), my guess would be entrenched interests made for a very difficult position for anyone wanting to change the ship's course.