Actually, it's pretty easy to setup such a system if you have the "right" tools. The most famous tool, Tweet Attack (link, but I am not affiliated or anything http://tweetattackspro.com/) can reveal itself super powerful. You can create a lot of accounts, make them chat together, tweet naturally as well as mentioning people, auto follow/unfollow. Those tools are really popular and if you know how to search you would find a lot of these kind of accounts. But don't worry, Twitter is aware of that and works to get all these accounts down. All the spammers get their accounts deleted pretty fast now, unless they are very careful.