Twitter's co-founder Jack Dorsey sent the first-ever tweet, on March 21st, 2006 at 3:50 pm. It said, "just setting up my twttr".
Since then, millions have set up accounts, using the site to share information with their followers, and friends.
Wednesday, "Happy Birthday Twitter' is a trending topic -- with users all around the world giving their praise for the site, all using less than 140 characters, of course.