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.

Answering a technical question takes more time and effort than making general comments.

I can bullshit for hours but answering even a few technical problems in a well written fashion is exhausting work.

You need to understand these are not only technical answers to technical questions, but also highly valuable ones because people must indicate that they "like" the answer.

Most even good answers get very little rep so the answers have to be pretty good to get that amount of rep on average.

I wonder how much rep, has to do with who decides to up vote? If you see Jon Skeet answer, people will up vote him more than the average user with 5k. This is a subjective opinion of course.