Juan Echeverria, a computer scientist at UCL, has published a paper on a network of 350,000 Twitter bots he calls the Star Wars botnet. Some of the accounts are used to fluff up follower numbers, send spam and boost interest in trending topics. From the paper:
A large number of Twitter users are bots. They can send spam, manipulate public opinion, and contaminate the Twitter API stream that underline so many research works.
