Legal status is frequently based on reasonableness. When you have a service with hundreds of millions of users, you already have a reasonable argument for "sign

Legal status is frequently based on reasonableness. When you have a service with hundreds of millions of users, you already have a reasonable argument for "significant" even if the lower bound of significance is fuzzy. If you then survey the users of a giant system regarding practical alternatives, and the answer is overwhelmingly "even if there were one, I couldn't switch to an alternative offering the same features and price, because it would lack the critically important feature that everyone uses this one," you have a reasonable, practical, realistic means of identifying some of the entities that are too powerful, relative to their users, to be allowed to just do whatever they want with prices, who they will do business with, and any number of other policies. If you want complete freedom to refuse to do business with users for whatever reason you want, charge anyone whatever you want and change as you like, and so on, you can have it as long as your users can reasonably exercise similar freedoms by doing business with alternatives. If network effects remove those options from your users, they should remove them from you, too.