Does anyone know of any communication platforms  which use a credit system or have a 'cost' attached to actions such as making a post or commenting? I am imagining something like Reddit or a forum where users have a balance, and actions have a cost which is charged against that balance. So if I have 100 credits and posting in r/whatever costs 2 credits/post and 1 credit/comment then that limits the amount of interaction in that sub.
I am wondering if a cost system like this would be useful for moderation or to promote high-value content, since it effectively turns the platform into a market. One effect of this system is that it would discourage low-value posts/replies/comments, because there is a cost associated with making a post, namely opportunity cost of posting something else later. Perhaps the credits are purchased with real-world currency, which I assume would amplify this effect?
I imagine a sustainable system would have some way to reward users of high-value content with more credit so they are incentivised and able to produce more content: maybe upvotes count as credit, or users can donate credit to each other?
 I hope this term is vague enough to encompass all forms of modern digital communication. I am curious about direct communication (email, WhatsApp, ...) as well as social media in its various forms (Reddit, Tildes, Twitter, ...), niche platforms (Letter), wikis, fora, and anything else under the sun.