A piece of US legislation could address this. For example, if a business that connects people to people—such as a social network, an instant messenger, etc.—grows larger than X users, it could be required to allow fully-featured third-party clients to exist, or it can’t operate.
(This is not dissimilar from how mobile providers are required to facilitate phone number transfers, so that you can switch to another provider and not lose your number.)
If big players are required to offer a fully featured API, and the user can switch freely, they would be incentivised to stop taking their money from the advertiser and charge the user directly, becoming a commodity like electricity.
That way you could switch between messaging providers freely, while using your preferred client app. Messaging services will be competing on which one is the most reliable, ethical, has the most complete API, and so on.