Twitter (unionsquareventures.com)

The Internet is largely a communications medium. It's also used for content distribution, commerce, and a number of other things. But the thing I do most on the Internet is communicate. Via email, Skype, instant messaging, blogging, commenting, and twittering. It makes sense. When you connect 700mm+ of the Earth's inhabitants to a global network, people are going to use it to talk to each other.