1) No one would bother creating these free services if they could not draw massive revenue streams out of it. If the returns disappear, it'd all be shut down.
2) If you create content on an online platform, you get paid in emotions you feel, the popularity you might get or the money you get through your own advertising if you are popular. If you contribute the data, you also benefit from a better service which is free.
P.S. I am thinking of building an RSS reader app with the built-in discussions. If it sounds interesting, please fill out a survey https://forms.gle/9V85Eb8PZnBXKaFr6