Force them to provide an openly accessible API to allow people to receive messages/event invites and send messages to the users of that platform. Then they can

Force them to provide an openly accessible API to allow people to receive messages/event invites and send messages to the users of that platform. Then they can choose to use a different social network without having to give up their connections with people who haven't jumped ship. This way the advantage of the network effect of popular social networks will disappear.