Twitter, Facebook, et. al. have the engineering resources to get creative and devise solutions (such as inlining advertising in such a way to make it computationally difficult to divorce it from desired content---YouTube, for example, has control of the video source and if necessary could start splicing ads directly into the video vended to users in a way indistinguishable from the content itself). The firms that can't afford to do that are small newspapers, individual developers, etc. They'll either pay their own way to put content up, GoFundMe a support base for their content, or give up and do something else.
The result will probably be a more centralized and perhaps less diverse world wide web. Content-agnostic advertising made a lot of room for "little ideas" to find footing. Without a similar model to that, web content defaults to a patronage model for every piece of content that isn't immediately monetizable. This is how the web worked before content-agnostic advertising (i.e. you only put up content if you were in the government, military, university, or paid someone with access to the backbone) and we could go back to that model, but it'd be a smaller web for it.