>> I'd be in favor of a maximum number of tweets per day, too.
A while back, I spent a few days hashing out a concept for a social network that would impose posting limits. The initial obvious approach is a single post per 24 hours, to really make each person think about whether hitting send is worth it. However, I cast this aside as being too restrictive.
Introducing the concept of categories: you can post once per 24 hours, but per user-defined category, up to some maximum category limit per account. You could post once about your cooking hobby, once about your programming hobby/career, once about the news, etc. Your followers could subscribe or unsubscribe to your individual categories; someone can opt to see your cooking and news posts, but ignore your posts about programming.
The concept starts to collapse at the "policing" stage. People would cheat by using all of their categories to spam a single topic, or to bypass followers' unsubscribing from a topic they just must be heard on. This necessitates a moderation system whereby followers could vote on posts, or report those which are miscategorized. With such a system in place, the possibility exists to increase the limit to 2-3 posts per day in categories for which someone receives overwhelmingly positive (or nonnegative) feedback. Dealing with users' and bots' vote manipulation is of course a never-ending battle.
The concept completely disintegrates when you consider who, as a publisher of content, would choose such a platform. A lot of people love sharing their every thought, meal, personality test, quiz, BuzzFeed article, etc. Such people would never opt in to a platform that limits their ability to do so - and why should they? Companies, celebrities, and other popular figures rightfully depend on being able to communicate whenever it suits them, so this concept is utterly useless to them.
Thus the market is tiny, to the point of being non-existent, for limiting the publishing side of the equation. This pushes the entire problem to the side of the readers/followers. We as consumers really just want to be able to intricately tailor what we see. I'm hoping we will soon have AI that can appropriately categorize posts, so I can filter out every single post having to do with personality tests and quizzes, BuzzFeed-like links, anything and everything having to do with US politics (I'm not American), animal abuse (I'm really tired of "save the animals"), and the constant stream of photos from new parents parading their children. I stopped using FaceBook entirely because their filtering options are just not adequate. Nor do I expect FaceBook to ever offer such filtering, as it would nullify all the mindless drivel that keeps their users engaged with their platform.
Lastly, I considered a monetization strategy whereby a user could pay (heavily) to post above their allocated limit. It sounds useful if you imagine people using it very rarely to push out that one extra gem that surely all followers would love to see. Of course, "pay to play" has no place in a social network designed to reduce the number of posts, and I couldn't come up with a solution to have my cake and eat it too. A "pay per post" strategy would be the foundation of a standalone concept - one for which no market exists.
tldr; A social network that limits publishers' ability to maximize engagement will never gain traction. What we need, as consumers of social networking, is the ability to perform advanced categorization and filtering of existing content. Don't get your hopes up - FaceBook and Twitter will never allow us to skip straight to the "good stuff", as viewing only a dozen posts a day would decimate their engagement stats, including ad views.