What is C++ used for in websites?

I read on Wikipedia that Facebook is written in PHP and C++. What would C++ be used for in Facebook?
I think that what it is used for is the server that facebook is on, examples of this may be for tasks such as encrypting things on their server and just maintaining facebook. php is used for the login form and other things that are dynamic. C++ has nothing to do with the design!
ok thank you :)
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