That is a good question, but it is extremely difficult to answer right now because some websites operate without following with the rules. Publishers are trying to get them shut down, but they pop up elsewhere.
It is difficult to measure their impact, but writers tell us about them. They see their work being copied, pirated by users who are difficult to catch, especially since those platforms are accessible from any country. It is not easy. That is why piracy should be considered very seriously in policies, which should find ways to fight it. The damage should be measured.