It's hard to imagine this today, but when Twitter was launched on March 21, 2006, it wasn't intended to be used for conversation. Say what?! Yep, its initial purpose was for status updates or brief, important messages. For example, "At Starbucks on Main." Or, "Accident on 5th + Lacy. Use Bristol exit." Founders did not incorporate any means of replying to someone's tweet, or retweeting someone's message. But it didn't take long before users of the nascent social media began adapting it to their needs and desires. One of these was, yes, being able to reply to someone's tweet. Another was to be able to catch a particular person's attention so they'd read your tweet -- sort of like being able to shout, "Hey John! Listen to this!" The way both problems were solved was through use of the @ symbol [source: Seward].
The first person to use the @ symbol in a tweet specifically to call out to another user (as opposed to the common practice of using it as a substitute for the word "at") is believed to be one Robert Andersen, who tweeted pal Buzz Anderson on Nov. 2, 2006, "@ buzz - you broke your thumb and youre still twittering? That's some serious devotion." Presumably, Buzz noticed the tweet.
The staff at Twitter certainly did. Six months later, on May 30, 2007, Twitter unveiled "@ replies" as an official feature. Rules then stipulated there could be no space in between the @ symbol and the person's name if you were trying to reply, plus the tweet had to start off with "@buzz" (or whoever you were responding to). So Andersen's usage -- he started with the @ symbol and buzz's name, but inserted a space in between them -- was incorrect in retrospect. The first accurate @ reply tweet was done by developer Neil Crosby on Nov. 23, 2006 [source: Seward]. Don't tell that to Andersen, though; if you go to his Twitter account home page, @rsa, his bio includes the lines, "Accidental sender of the first Twitter @-reply. Sorry."
While Twitter made the @ symbol an official way to answer someone's tweet, use of the symbol with someone's name within the tweet -- not just at the beginning -- was also a way to call out to that person.