Also remember about Sourceforge. I guess some people could have been in the mood to write gratitude letters to them at some point in the past.

(Does not mean that GitHub is "bad" or be useful for some free software projects right now, just that from where I am I don't think there should be a special reason to thank such a company in the name of "Open Source" -- at least also thank Linus Torvalds to begin with!...)