It's been a few years since I've been on Facebook, but doesn't facebook show your comments to your friends by default? It's not like Reddit or Hacker News where everyone sees the same comments, is it?

Yes, you will see comments from friends on posts like these. Even if they have a million comments it will come into your feed with a friends replies visible.

And I think that's why these are so viral. When you see a question and your friend's answer, it makes you feel like you're responding as well.

That or the post is some variation of "tag someone that does X", or the one I've recently noticed "tag your favourite celebrity and see if they'll respond". Random pages I've no connection to are now popping into my feed because some page I follow responded to someone that tagged them under one of those. Completely pointless.

There are millions of fake social profiles on Facebook, LinkedIn, Twitter, etc that use fake generated profile photos, friend networks among both themselves and gullible real users, and automated markov chain style comments on popular content. It's a self-perpetuating system and an effective way to see inside the walled gardens.

How do you know this and where can I read more

facebook's commenting interface, showing only the few latest comments and discouraging threading is aiming to create the largest number of duplicate comments with immediate (and thus outrageous) reactions.