As of May 23th, 2018 much of this seems to have changed.
From the terms of service
I. Additional Terms for GitHub Pages
Short version: The GitHub Pages hosting service is subject to certain rules, in addition to the rest of the Terms.
Each GitHub account comes with access to the GitHub Pages static hosting service. This hosting service is intended to host static web pages for GitHub users. GitHub Pages are subject to some specific bandwidth and usage limits, and may not be appropriate for some high-bandwidth uses. Please see our GitHub Pages guidelines for more information.
GitHub reserves the right at all times to reclaim any GitHub subdomain without liability.
K. Advertising on GitHub
Short version: We do not generally prohibit use of GitHub for advertising. However, we expect our users to follow certain limitations, so GitHub does not become a spam haven. No one wants that.
1. GitHub Pages
We offer Pages sites primarily as a showcase for personal and organizational projects. Some monetization efforts are permitted on Pages, such as donation buttons and crowdfunding links.
2. GitHub Repositories
GitHub repositories are intended to host Content. You may include static images, links, and promotional text in the README documents associated with your repositories, but they must be related to the project you are hosting on GitHub.
You may not advertise in other Users' repositories, such as by posting monetized or excessive bulk content in issues.
3. Spamming and Inappropriate Use of GitHub