1) It allows you to instantly connect to a huge market and promote your product, brand, etc..
2) It's the fastest and simplest way to stay close to your prospects and customers.
3) There is no "noise" created by boring people on it since "friendships" are not mutual.
4) Twitter made huge success from the mobile device users.
5) In February 2009, Twitter had a monthly growth (of users) of over 1300 percent - several times more than Facebook.
6) Twitter is a broadcasting Social Network that allows you to reach to a vast market with a single tweet.
Marketing is about numbers, and these support Twitter.