Facebook also uses Haskell (several well-known members of the Haskell community work there).
To transform PHP code, it seems [1]. That's not quite the same as the backend and infrastructure of Facebook and Google.
I'm not trying to start a flame war or anything, just pointing out that C++ is good at some things you cannot really do with friendlier languages.
Not only [1].
1: http://www.reddit.com/r/haskell/comments/1le4y5/the_haxl_pro...
However, C++ tries to be both, that's what's special about it. It's also arguably the cause of most of its problems and it's complexity, but that is what makes it powerful and useful - for certain applications.
> The main micro blogging site I use is a mixture of scala, java, with ruby/iOS for the front end.
Twitter seems to use Scala for most of their backend (after growing too large for Ruby), but Google and Facebook (arguably operating at larger scale than Twitter) mostly use C++ there AFAIK.