https://www.techdirt.com/articles/20211023/15545647803/trump-given-30-days-to-have-his-social-media-site-comply-with-open-source-license.shtml

Plenty of people have raised concerns that Donald Trump's sketchy new social media site, Truth Social, is just a lightly reskinned Mastodon, which is violating Mastodon's fairly strict AGPLv3 license. As we had previously discussed, the aggressive (and sloppy) terms of service for the site claim that the code is proprietary, and even claims that "all source code, databases, functionality, software, website designs, audio, video, text, photographs, and graphics on the Site (collectively, the “Content”) and the trademarks, service marks, and logos contained therein (the “Marks”) are owned or controlled by us or licensed to us..."

Of course, part of the reason that Mastodon uses such a license is to encourage others to take the code and build on it if they abide by the terms of the license. And the nature of Mastodon's license is that if you use it, you must make the complete source code available of what you build with it. The key bit of the license: