Note that Obama's birth was a conspiracy around 2008, before 8chan existed. I would argue that "the Donald" on reddit was a driver of Trump's online popularity. This argument is not for striking down individual websites as much as it is for pervasive rumor-quashing.

One could look toward China's internet policy to measure the success of playing "whack-a-mole" at the rumor-level. Propaganda there still gets made and spread, it just stays online for minutes at a time before deletion, and so by the time people see it they have no way of asking for clarification on its truth.