>The ability to close yourself off in a bubble where you can't be reached by opposing viewpoints is relinquishing your duties as a democratic citizen.
This is only true if each side has a valid and reasonable difference of opinion. This is not true for hate speech and other propaganda. For example, it's not an "opposing viewpoint" to say that people with dark skin are subhuman.
Yes, which is why many European countries have laws against hate speech, although the US doesn't.
But political advertising doesn't equal hate speech. Many people currently support the idea of Facebook and Twitter prohibiting hate speech, political or not.
That has nothing to do with being able to turn off political speech in general. It's a totally separate issue.
As for propaganda, that's in the eye of the beholder. Which is why we generally frown on censorship, because one person's propaganda is another person's deeply felt, legitimate political viewpoint.
How much political power does the Nation of Islam have? How many members of congress identify as members of the Nation of Islam?
Wikipedia says they have 20,000–50,000 members total. You're not making a meaningful or valid comparison by comparing the beliefs of <50,000 people to the the incredible power of structured prejudice in the USA.
This is only true if each side has a valid and reasonable difference of opinion. This is not true for hate speech and other propaganda. For example, it's not an "opposing viewpoint" to say that people with dark skin are subhuman.
https://en.wikipedia.org/wiki/Propaganda_(book)