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Because it allows you to 'broadcast' your messages without explicitly directing it to anyone in particular, its different from conventional IM. Because you can 'follow' people you might not even know for information, it gives it a somewhat 'rss' flavor. It sometimes takes the role of conventional IM too... and sometimes, it serves as a tool to update people of the minutae of your life (the part that I don't like-- considering the fact that strangers, and COMPANIES would be interested in that data, and might be able to interpret it well. Be careful of your privacy, and twitter is a useful communication tool with its own niche-- I think I'm beginning to see why it is popular.