I joined two years after they started which is earlier than I realised. And apparently we're both relatively early adopters given that there's supposedly >100,000,000 users now.
I have a two-letter github username and I wasn't even an early adopter. I only bothered creating an account when it was clear that Github was more than a fad and a few of the projects I contributed to moved there. I tried just my initials and it worked.
Having a two-letter username is less exciting than simply a common word or name, though. Once in a while I get mentioned by accident in issues, or added to some organization. Twice I have had someone beg me to give it to them under the rationalization that it was "really cool" and I wasn't really "doing anything" with it...
EDIT: I just looked up my ID and apparently none of the first half-million people to sign up for a github account tried to brute-force the small-username space.
EDIT2: There are a few single-letter usernames with IDs in the millions, so I wonder if those are somehow "kept in reserve" by github admins and given out to friends or what's the story there.
There are some more three-character user names if you count dashes and digits, which are also allowed in GitHub usernames. Although I think the first one has to be a letter of the alphabet.