Paid content on Facebook is indeed labeled as an ad or sponsored content. The problem is that fake news can still generated likes, shares, and get into the coveted "trending" topics area. Which gets clicks out to the fake news site, where the fake news proprietor can generate ad revenue however he/she likes. It is a step forward to prevent them from using Google or Facebook ad networks, but I suspect there are still a lot of ad networks for these fake news sites to use. At that point, they are not using Google or Facebook for direct revenue, rather for simple "grassroots" promotion.