How does servers satisfy increasing online storage?

Websites like facebook, twitter, and other sites like Yahoo! Answers store the data entered by the people who use their websites into their own servers. Now there must be more than millions of people who use such websites. So how do these companies manage their online storage? In years to come more people will join the internet and enter data onto those servers. Now the size of the earth is limited. So how will those companies manage to store the data? On which part of the earth will they store their hard disks and other storage drives? I hope you understood my question.