AOL Instant Messenger (which was super popular at the time) had this format. You could choose your own screen name, maybe he wanted 02 instead of 01 for whatever reason.
Presumably because he registered his nick in 2002 (which was a popular "naming scheme" at the time). thinkcomp should tweet someone like @pinboard, @nitashatiku, @karaswisher etc.
Why is Zuckerberg's nick 02? Who is 01?