Congrats, Twitter. Another product that was mocked as a "toy" becomes revolutionary and finally going to raise money on public market. I use twitter everyday and it is my primary information network. I'm sure they will be successful!
Sharing links with friends, discovering new content, getting news I'm interested in. (I use Twitter, but not Facebook.)
Discovering new content. I follow influential people in the areas that I'm interested in. And they are always sharing interesting links/insights/opinions. And I also share some!