People like being told that they are smart, indirectly. If someone posts a "hard" math "problem" and the viewer can solve it... that indirectly means that the viewer is smarter than the poster. Which viewers love.
Did you read the comments from the guy that left facebook? It was like reading YouTube comments. People were flaming him from their professional accounts.
It's like the reverse of the /g/ or /sci/ meme with a picture of Makise Kurisu saying "You should be able to solve this" accompanied by a laughably difficult problem.