Hashtags are a community-driven convention for adding additional context and metadata to your tweets, similar to how tags work for blogs. Anyone can create a hashtag. To start a hashtag, just prefix any word with the pound symbol: #wth Hashtags are a method for grouping related tweets. If you are going to a conference, there is probably a designated hashtag. When you add that hashtag to your tweets, it makes it easier for other attendees to see what you are saying about the event. Hashtags are also helpful in finding other users who have similar interests.
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Hashtags have become a popular way of identifying micro messages about a given topic. #hc09, for example, was a hashtag proposed for supporting the Obama health care reforms. But there is not list of hashtags. This site is a volunteer effort to identify what is being communicated with the hashtag, and how frequently it is being used. It is an imperfect list of hashtags in use, but it is one step in that direction.