Twitter previously used some stock bird art, but the logo was just the bubble letters. Now they have an original bird logo. But 3rd party Twitter stuff can use arbitrary bird art, or maybe the old stock one since it's unclear who owns it.
I think a good solution would be a new open license logo, like the Minefield logo as opposed to the restricted Firefox logo.
it would be a nice way of everybody having a common "icon", much like twitter has the bird.
I know people can link to github, but that's not always possible (mobile apps?) even though people use github..