[jwatzman]

FB engineer here. We did migrate slowly, to Hack http://hacklang.org/. It's a typed dialect of PHP, notably with full backwards compatibility, so we didn't have to rewrite all of our code as we converted over and added types. I haven't looked at the stats in a while, but last I checked, well over 99% of our files were marked as having some type information, and a very significant proportion of that was becoming fully statically typed. Basically all new code since the end of 2013 has been mostly, if not completely, statically typed.

I'm actually giving a talk at OOPSLA this week about how we went about actually doing the conversion (everything from how we approached designing the system to how we convinced engineers to use it), if you or anyone reading this happenes to be going. http://2015.splashcon.org/event/splash2015-splash-i-josh-wat... I think it will also be recorded.