True, "dickheads" invading subreddits and upvoting each other, or more generally new people coming in whose preferences may differ from the existing community,

True, "dickheads" invading subreddits and upvoting each other, or more generally new people coming in whose preferences may differ from the existing community, is one way the voting algorithm can fail to naturally enforce quality, but it's not the only one. As I see it, most (existing) users of a given subreddit appreciate the improvement in quality that results from moderation, yet when some of the lazier instincts of those same people are pooled by mass low-effort voting, funny or memetic or clickbait or flamebait but ultimately low-quality posts tend to rise. For example, on this site, posts whose titles contain "NSA" and some other keyboards are or were penalized in the ranking, because people here, including many of the most famous users on the site, just love to read and talk about the NSA - which isn't itself bad, but since it's so easy and compelling to instinctively hit the upvote arrow on such posts, it tends to get too common and repetitive, so the algorithm taps (tapped?) the brakes. (At least, that's my interpretation of the reason.) As another example, /r/smashbros is a big subreddit for the Super Smash Bros. competitive community, and it's often happened that the moderators banned X type of posts (comics, non-competitive gifs) because a lot of people were complaining about it and there seemed to be consensus against - but that didn't mean the posts didn't get upvotes. You could chalk that up to a difference between the vocal minority and the many lurkers, but I don't think that's the full story. Admittedly, I've strayed pretty far from the sort of things that can happen on Twitter. After all, it's not like any feature of the site would discourage me from following 100 people who all constantly post about the NSA, and voting is different from posting. But I think there's a similar general principle when it comes to length, the one important form of "moderation" the site actually has: a user might get annoyed at the overall phenomenon of the length of the posts in their timeline, but it's difficult to exert social pressure on everyone to keep length down, and really easy to feel like one cannot say what one wants in 140 characters, whether or not this is true, if one is not actually forced to make it fit. Also, the aforementioned general case of splits in the community does apply to Twitter. You can unfollow dickheads, but if lots of popular users are in the subgroup that actually prefers long posts, those users who prefer short ones will have a hard time keeping their streams "clean" without feeling like they're missing out. Of course, if one considers the Internet as a whole, one could say that following long-post users is the equivalent of participating in social websites that allow long posts, but unlike the discrete universes of different sites, or even of different subreddits (in the sense that a given post or comment goes only to a single subreddit), Twitter's network allows, and indeed is designed for, cliques to partially overlap and mix, making it harder to establish different "spaces" with different rules, social customs, culture.