> Make every service with more than, say, a million users required to offer federation. There must be an API, and everybody must be able to get their data in and out free of charge, and there should be no limitations (except for fighting spam).

I totally agree. Setting a minimum of users would also help the controlling of the system and the startups getting on their feet immensely.

> "You may use this protocol / implementation free of charge in any way you like (source closed or open) under one condition: you have to offer federation."

I'm not working on a protocol but I wonder if such a license already exists. I would financially contribute if one were to hire a bunch of lawyers to write one.