After joining GitHub in February 2014, I was stuck with a long username. Fortunately, I didn’t have to settle for it.
Now, I have @tmm.
GitHub is one of the few sites that has a policy of releasing dormant usernames, and luckily there are plenty of them!
First, I looked for users that joined GitHub early on (2011 and before) and had little to no publicly activity (commits, repos, and followers/following). Then, all I had to do was contact GitHub Support with the username I wanted.
And it worked.
Changing your username has unintended side effects.