GPL would be good if it is a self contained library. If anyone would use it, it would be small portions of it, but GPL makes it completely useless. You can't contaminate anything with it. We'll stare at it, that's about it.

That makes me think, this is actually a good call. Twitter can claim that they have complete transparency while not allowing anyone to touch their code (because it is GPL). "Anyone" being future competitors. If it was BSD licensed, it'd be tremendously useful in building a Twitter competitor (on paper, you still need network effects, I am just spitballing to make a point).