It would be a pretty easy legal definition. Start with services that have over 100 million users: https://en.wikipedia.org/wiki/List_of_social_platforms_with_a

It would be a pretty easy legal definition. Start with services that have over 100 million users: https://en.wikipedia.org/wiki/List_of_social_platforms_with_at_least_100_million_active_users https://en.wikipedia.org/wiki/List_of_social_platforms_with_... And exclude messaging services eg. Telegram, or platforms which are focused on technical or professional knowledge sharing (eg. Quora, LinkedIn, Wikipedia). I think the risk would be conglomeration - platforms would partner up so that the minimum fee could give access to multiple services. You'd also want to avoid the fee from just becoming a 'token' with advertising still driving 90%+ of revenue. Old-fashioned antitrust enforcement might still be necessary (eg. separating Facebook from Instagram and Whatsapp), as well as setting a maximum % of gross revenue that can come from advertising, maybe 75%.