Complete agreement. Furthermore, bad actors out there will abuse "ugc": they will mark terabytes of robot-generated content as "user-generated". It will not be possible to rely on it as an indication of organic content or anything of the sort. Search engines won't be able to use that for ranking, for instance.
If I know that I'm traversing and processing facebook/twitter/reddit links, doesn't "nofollow" already indicate user generated content? (Even if some "nofollow" links do not do that, I can probably separate those based on their position in the documents.)
If you're going to treat specific sites specially, then go in all the way and be prepared to have your logic understand any/all aspects of their structure. Or else, don't bother.
