Are there somewhat popular social networks that require commenters and posters to license their work under a free license (e.g. CC-BY-SA-4.0)? Then anyone would be free to create a "clone" of the website and even make derivatives, all without breaking copyright law.

There are many wikis like this (e.g. Wikipedia) and at least one forum (trisquel.info), but I don't know of anything for more general purposes. Even most fediverse sites, which implicitly grant permission for some specific kinds of mirroring, don't usually have a free license that allows redistribution in completely different contexts, as far as I know.

YouTube lets users select license for their video uploads, and Creative Commons Attribution is one of the available choices but not the default. https://support.google.com/youtube/answer/2797468?hl=en

PeerTube also lets users specify license for the videos they upload, including many different Creative Commons licenses. Administrators can configure default license values for videos uploaded to their specific PeerTube server since last year. https://joinpeertube.org/news/release-7.3

The only issue I imagine with defaulting to a Creative Commons or other kind of permissive license is that the users uploading content might be unaware or not properly understand what it means, if they themselves did not actively opt in to use that particular license.