I think most of you guys are missing a crucial thing here -- how decisions get made. Facebook had two options: 1.) Rewrite all the PHP code in Java/Ruby/C++/whatever 2.) Write HHVM/Hack/etc to transparently convert the existing PHP code.

Option #2 ended up getting a lot of publicity for the engineers. If they chose option #1, not only is it a lot of hard boring and possibly error prone work, there is no chance for any of the engineers to get the type of publicity they are currently getting. All in all, engineers tend to work on whatever will make them get noticed, not necessarily the better technical choice nor what is in the best interest of the company... this is especially true in companies like Facebook and Google where there are a lot of very smart engineers doing relatively mundane work. So there you have it Hack/HHVM are all just publicity stunts, the more you feed it, the happier their PR gets.