There are two things that encourage sites to use
rel=nofollow attributes on untrusted links:
- Google requires it on paid (advertising) links that Googlebot can find. Google will penalize sites that don't nofollow their paid links by making them rank worse in the search results.
- When nofollow is used, spammers have a less of an incentive to post on the site. Many spammers post links that they think will improve their search engine rankings. When nofollow is used, the links won't help them.
If the page with the link can't be crawled there is no advantage to using a
rel=nofollow link attribute. Google won't penalize for non-crawled pages. Since those links are already not going to influence rankings, spammers may not post their anyway.
Facebook doesn't care about search engine penalties because it doesn't get very much traffic from Google. Most Facebook users visit periodically or from reminders from apps and email. I don't see any way that nofollow is enforced on Facebook by Google. That being said, Facebook does use nofollow on external links in posts as well as in advetising in the Facebook feed. I've never seen content posted publicly on Facebook show up in search engines, but I suppose it could happen and Facebook could be paying attention to that.
Quora does care about its search engine ranking. It would not be happy if Google were to de-rank its pages because it failed to use nofollow on paid links.