The first app I know to use "@" to address people is Twitter and then it caught on. I know that "@" was used in Usenet as a replacement for the word "at", but not to refer to specific users to my knowledge.
Edit: I am not referring to just using the @ at symbol in general. I am aware of this article: http://en.wikipedia.org/wiki/At_sign
I am asking for the first time the symbol was used to refer to a specific person or user. For example, @Jane and NOT firstname.lastname@example.org.
Did Twitter invent this usage when it was founded in 2006? And if so, who made that decision?