As with most services that become popular dodgy characters eventually show up to try and take advantage of the new system. Twitter has developed safeguards to shut down spammers and protects users from receiving unsolicited messages. You should be aware of the limitations that are in place so that you do not accidentally appear to be a spammer and risk having your account shut down by twitter.
Following: Automated pro-active following and automated un-following are not allowed. If an account seems interesting, feel free to follow it! When you follow someone, they will often take the time to check our your profile and recent Tweets. If some accounts are aggressively or indiscriminately following hundreds of accounts just to get attention, it can really bother other users and make Twitter a less-nice place to hang out.
@Replies: You can direct a Tweet at a specific Twitter user using @replies and mentions. The @reply feature is intended to make communication between users easier, so please don't abuse it by posting duplicated, unsolicited @replies to lots of users' replies tabs. This is considered spam behavior!
Trending Topics: Participating in Twitter trending topics are a great way to join a world-wide public conversation. Our Trending Topics help page has information on how to join in. Please don't post unrelated Tweets to trending topics in order to get attention—your account may be suspended!
Search (and contests!): Twitter Search is a powerful way to find out what folks are talking about, right now. We may filter accounts from search if they're degrading the search experience for other users (by posting lots of duplicate links, for example). Our Search Troubleshooting Page has all the best practices for remaining in search. If you're wanting to enter lots of contests on Twitter, please check out that page, and be careful not to post duplicate contest Tweets. Folks wanting to run a contest on Twitter should check our Contest Guidelines for some rules and helpful tips.
More details can be found here: http://help.twitter.com/forums/10711/entries/69214
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