Github recently drafted an update to their Terms Of Service. The new TOS is potentially very bad for copylefted Free Software. It potentially neuters it entirely, so GPL licensed software hosted on Github has an implicit BSD-like license. I'll leave the full analysis to the lawyers, but see Thorsten's analysis.
I contacted Github about this weeks ago, and received only an anodyne response. The Free Software Foundation was also talking with them about it. It seems that Github doesn't care or has some reason to want to effectively neuter copyleft software licenses.
The possibility that a Github TOS change could force a change to the license of your software hosted there, and that it's complicated enough that I'd have to hire a lawyer to know for sure, makes Github not worth bothering to use.
Github's value proposition was never very high for me, and went negative now.
I am deleting my repositories from Github at this time. If you used the Github mirrors for git-annex, propellor, ikiwiki, etckeeper, myrepos, click on the links for the non-Github repos (git.joeyh.name also has mirrors). Also, github-backup has a new website and repository off of github. (There's an oncoming severe weather event here, so it may take some time before I get everything deleted and cleaned up.)
[Some commits to git-annex were pushed to Github this morning by an automated system, but I had NOT accepted their new TOS at that point, and explicitly do NOT give Github or anyone any rights to git-annex not granted by its GPL and AGPL licenses.]