1) It's hard to define: Sophisticated audience measurement companies, like ComScore and SimilarWeb, are agnostic in their definition of what is considered fake news.

2) Which makes it hard to measure: SimilarWeb spokesman Ariel Rosenstein says the site can't always draw accurate conclusions about a website's overall reach based off inconsistent traffic patterns from sites frequently changing domains and subdomains.

3) It's still easy to monetize. Fake news sites can buy advertising through self-serving platforms once they receive enough impressions. Third-party ad-servers, like Google's AdSense, have implemented policies against fake news advertisers, but they have to be wary of over-characterizing which sites are considered real or fake.

4) Because regulators aren't acting: The FTC has the power to penalize sites that mislead consumers or affect consumer behavior. But the Commission rarely enforces the Act on accounts of libel that don't affect consumer behavior.

5) And it doesn't need Facebook or Google to survive: As Digiday notes, ad networks that distribute content to publishers at a low cost are also struggling to keep fake news sites off their distribution channels.