Some Reddit users have noticed that they can’t sign-in to their accounts anymore on mobile, as the option to do so has been removed for them. Reddit has made it difficult in the past for users to access the site on mobile in browsers. The company’s main intention is to get users to use the official Reddit application instead.
Other than Wikipedia, which asks for donations pretty prominently every time you use it and has some substantial corporate backers, I don’t know of any major projects that’re wholly backed by donations. Blender and Krita I guess? None of which are really social media though