It is a substantial challenge, but we had the same debate with the rise of television when we went from three or four broadcast channels to 200 channels—that the wash of information would make it impossible to differentiate credibility and quality. Over time, people developed new schema for how to sort, categorize, and judge the quality and credibility of sources on television. The same thing can happen with the Internet.
I would also emphasize that you don't need to have a Ph.D. and do a dissertation on every source that comes in to evaluate what you think about it; you need to have some quick and easy ways to evaluate how credible you find something. Those things can be taught in civics classes. They can be taught relatively broadly and in a content-neutral way so that people are simply equipped with the skills to judge when and how they ought to apply more cognitive energy to evaluate the credibility or the quality of the source.