There is comedic fake news, fakier news, and then there is serious fake news.
While occasionally people are duped by The Onion, the site itself doesn't pretend to be posting real stories. Believing this is akin to believing the fakier news, which is basically the tabloids of old. Lizard people, celebrity "news" and gossip and the like. A certain subset of folks have always gone for that stuff, but I like to think that more of them read it for the giggles. The main reason any of this gets into mainstream is lack of fact checking by the real news folks.
But the other breed, the serious fake news, is the real problem. Even otherwise intelligent folks can't really sort out if they are real or not because it is just so freaking plausible. Their goal is to be believed, and reaffirm some suspicions and beliefs people have. Even worse, some of these sites will mix in factual stuff with their fakes. These are the ones that, "seem to be seeing this as an opportunity for a power play". In a sense, any news organisation that caters to any political party - fox news to an extent, and the more insidious players (occupydemocrats, for example) - are guilty of just this. Twist, lie, and misrepresent are part of the game.