Then you might have a problem since you don't have the rights to transfer that license to twitter, and I can't imagine the infectious nature of GPL would apply in this context. I suspect given the format of twitter, it would likely fall under fair use for most code snippets, but certainly you'd be muddying the chain of ownership. My guess is that something like this would never make it to court. There'd be some legal letters sent around, and anyone involved would quickly back down. No one wants to go to court to have something like this ruled on over 120 characters. If it was, you can bet it'd be a very strategic tweet that was intended to force everyone into a courtroom (and I can't even imagine what that would look like).