How does mega websites benefit from operating their own hosting servers instead renting them?

[Harvey]

most of us can just buy a domain name attached with a monthly payment on a hosting.
but if your service goes uber mega scale, why would it be more advantageous for websites like myspace or facebook to operate their actual hardware-servers and attach hosting to their domain name?

[Andrew Rempel]

Due to the mass amounts of hits and data, they must have their own servers and even own data centers to handle the traffic. Shared hosting that us regular guys use only works for small sites. The more your site grows, the more servers you need.

[Taylor]

Google & Facebook are much bigger than most hosting providers, meaning they can leverage purchasing power when buying servers.

Also it's important to note that only huge companies save by bringing hosting in house. Some companies getting over 5million+ visitors/day still use 3rd party hosting providers.