Imagine if Facebook or Google started doing this.

"Oh, you're friends with someone who says X? Welp, you must be a Bad User and your posts will receive a lot less visibility and the validation that you desire. But if you were to stop interacting with your Bad Friend, you could become a Good User again."

And oops, "Bad Friend" no longer gets invited to real-life parties because most people still consider FB the most convenient way to organize them and people only rarely remember to forward the invites.

It already does work like this with Facebook and Twitter controlling which posts are displayed for you.

The enforcement works better when you aren’t aware of it. Being covert means you don’t protest so loudly.

Isn't that the same online but with greater enforcement?

It's less severe, sure.