A few years ago my friend Jack built a cute little application. It was a text message multiplexer. You could send it a text message and it would send that message to all of your friends. You signed up using your phone number and gave it your name. It was somewhere between addictive and annoying but completely social, since basically all of the users were our friends. We mostly used it as a free-form Dodgeball, to work out when friends were out at bars and inevitably they added the ability to send a message directly to a contact. There were no usernames so twttr would cleverly work out who you meant based on the first name you supplied and your contacts list. This never worked right, they added usernames and now I'm @ian. Unfortunately then the whole @reply thing happened and people do just use their friends first names. Look at how many people use @ian - most of them are not talking to me, but other Ians.
Facebook resisted giving people anything other than a free-form name and a numeric user ID for the longest time. They finally gave in and let people pick vanity URLs but still refuse to make that URL useful for anything but getting to your profile. When they added @ support they pop up UI to autocomplete friends names from your contacts list. It works really well, but it depends on a rich message composition UI, something that's not possible on the simple mobile devices that twitter was targeting.
I wonder how the next site will approach this.