But as Youtube has walked into the door, Twitter has closed off, taking claim to the fact that Flattr receives 10% of the donated money moving through their service.
The actual clause goes,
IV. Commercial Use, 2C. Advertising Around Twitter Content
Your advertisements cannot resemble or reasonably be confused by users as a Tweet. For example, ads cannot have Tweet actions like follow, retweet, favorite, and reply. And you cannot sell or receive compensation for Tweet actions or the placement of Tweet actions on your Service.
And Twitter has specifically told Flattr, “Our API Terms of Service state that you cannot sell or receive compensation for Tweet actions or the placement of Tweet actions on your Service. This includes compensation attached to a Tweet Action sent to either a service or through a service to another user.”
This seems like a silly interpretation of their terms of service, as the clause seems to be focused on stopping companies from selling retweets and followers. But if anything, Flattr is providing encouragement for creators to put good content on Twitter, and encourages users to use the standard Twitter UI.
Hopefully Twitter will turn around here