Also:
> all ads are supposed to contain the word “sponsored” as part of a mandatory disclosure, so users can distinguish between ads and their friends’ posts. Our tool recognized ads by searching for that word. Last year, Facebook added invisible letters to the HTML code of the site. So, to a computer, the word registered as “SpSonSsoSredS.” Later, it also added an invisible “Sponsored” disclosure to posts from your friends.
https://www.propublica.org/article/facebook-blocks-ad-transp...