The curiously absent question is why there should be "social media" in the first place, and 'media' in general.
Not why there is - it's like free cocain - but why it should exist.
This is a political question.
The justification (ethical, moral, political, philosophical) for the existence of low cost/free communications between arbitrarily distant people needs to be questioned.
If you think that it's obvious, think again, and try not to factor in the allure of spreading your own pathetic output to billions with a keystroke.