Too many people make the mistake of believing twitter is all about the follower numbers. Yes, having followers is a good thing but if theyre already following you then they know you exist and either, are already customers or arent interested in purchasing.
Good use of hashtags is the answer and ensuring you engage your audience, theres a good thread in the Education forum about how to use it properly...add a good automated system to it (theres a good thread in the Business Services forum on that too
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Im interested to know though, how do you know you 'hardly ever make sales from twitter'?