What do you mean by "fake news"?
Is it yellow journalism? Right propaganda, left propaganda? Articles from the perspective of the skeptics, from the alt-right, Neo-Nazi's, communists, cultural Marxists or from radical feminists? Something else?
The problem is that people aren't able to accurately judge when they are inside a low-trust environment when using one of their many digital devices.
People are watching their neighbors return from the mysterious Facebook fog spouting fake-news bullshit like it's gospel. Then they themselves blithely enter the fog with the reasoning that they would never get taken in by something so crude and obvious.
If you've ever watched a shell game in a big city that fallacy will be familiar. You watch a frustrated victim keep losing and notice a subtle thing the scammer is doing to cheat. You then place your own bet and choose the shell where the cheating scammer inconspicuously tried to move the ball.
Then you lose, too, because you didn't consider the possibility that the scammer had yet another trick prepared just for people like you who think you're smart enough to figure out the game.