Solving problems for random people on StackOverflow helps them (the user, the corporation, and StackOverflow) make cash while not compensating you, and it
also requires relevant knowledge and effort to communicate concisely and correctly.
On the other hand, people on Hacker News, Facebook, Tumblr, /. and so on don't actually have to know anything, and their contributions only make money for the host company, and barely so in some cases. Reddit, as an example, isn't really profitable because of its average user.
I contribute to similar things, myself, pretty frequently, but let's not pretend like social media and collaborative knowledge projects are completely equivalent and that effort spent is transitive.