Github has a contact form: https://github.com/contact - Here's the love letter that I just sent them if you need something to copy-paste.
> The internet just told me about the updated ToS and its potentially disastrous effects on the legal certainty of thousands, if not millions of open source contributors. I demand you adjust your ToS to ensure that users can be safe that they will not get into legal trouble by sharing open-source code and artifacts on Github.com. If you fail to provide me with sufficient legal certainty in this matter, I'm prepared to move my source code to other providers or to my private premises.
> Since my employer, $name, is also a paying user of Github.com, I will also refer to my employer's legal department to check which legal concerns could arise from my continued use of Github.com for work purposes, and take appropriate action.