Facebook unveils Hack, a faster programming language to power the social network

[Adam D. Ruppe]

The reason for using their hack is probably legacy PHP code and 
experience carrying over; they can bring their existing staff and 
code to the new compiler then start bolting on new features one 
by one. Same reason they did the whole HipHop thing - trying to 
put some lipstick on PHP and now they're trying to polish it, one 
step at a time.