When Twitter first launched in 2006, the service’s modus operandi was to offer a blogging platform tiny enough that it could be done by mobile (dumb) phones. The 140-character limit Twitter imposes upon tweets to this day is a relic from this time, in which any tweet needed to be short enough that it could squeeze inside a 160-character SMS text message. Consequently, Twitter was much more no-frills back then than it is now. In fact, when the service launched, it didn’t even have a reply mechanism. If you wanted to reply to someone’s tweet, you just sort of responded into the ether in any way you hoped might catch another user’s attention.
The @ symbol’s adoption as Twitter’s accepted reply mechanism happened organically, and took about eight months. The first use of an @ reply can be traced to Thanksgiving Day, 2006, when a couple of Yahoo UK programmers named Ben Darlow and Neil Crosby started using it (as they wrote at the time) as a “pseudo-syntax to let a Follower on twitter know that you’re directing a comment at them.” Just two months later, the @ reply was the universal Twitter reply mechanism, and now, you can type @ and follow it with the name of pretty much anyone on any social network to direct a reply to them.
It's interesting how many social innovations have simple origins. It gives hope for more.