A site like Facebook isn't computing the value of pi or calculating jump coordinates for the Galactica, things that would benefit from a more efficient implementation. I don't know anything about Facebook's site, but many web applications use the application layer to essentially pull data from and send data to databases. Parsing data back and forth typically isn't sped up with C++ or even C that much compared to PHP. All many PHP sites do is draw HTML, and a lot of SELECT, INSERT, UPDATE, and DELETE queries. Hard to imagine C++ having a big impact on that, certainly not 10:1.