USA asking social-network password

TL;DR: it’s stupid, it’s often the worse “I have nothing to hide” thing and a (dis-united) state wants to to it. A password is meant to be hidden, even/specially to governments.

Other questions

What is a social network, is it what’s allowing humans to communicate or more specific thing like Twitter or Facebook

How can you verify that you gave all your social accounts, or even not a fake account which is created just for that (like recycling a _ebook bot)