...the @ sign followed by a username is used for mentioning or replying to other users.
IT MEANS when you put @ the a twitter user name on twitter it comes up as a link when you post it, so people can click on the username and go straight to the users page. Sorta like a cheat.
It's fairly obvious that that's not true, unless you consider Twitter a new language in which "at" means something other than "at."
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