Yes, a LinkedIn shirt would be lame, because everyone uses that service, and (in my perception) people who talk about leveraging synergy were on it before geeks

Yes, a LinkedIn shirt would be lame, because everyone uses that service, and (in my perception) people who talk about leveraging synergy were on it before geeks were. So it's neither a cool group nor an exclusive one. Whereas a Github t-shirt says, to other coders, "I am a coder who works with open source." And to non-coders, it means nothing. Coolness is mostly about exclusion, I think. Or to put it more nicely, it's about identifying with people you like. So if a coder can honestly say "I think Brand X is awesome, and those whom I want to impress also think that," he/she might wear a Brand X shirt to a conference. Otherwise, no.