The fact that these companies make a profit (Author Solutions, Inc. to the tune of about $ 100 million a year) speaks to the number of authors they attract with their sophisticated marketing programs and follow - you - everywhere web ads.
But, that last part you said... There is a huge difference between a single text ad on the web version of Miiverse off to the side (which would've otherwise been empty space anyway) and having adsEVERYWHERE.