Just force them to use open standards and for it to be easy for people to move platform whilst keeping all the connections to their profile: the power of such social media entities is that people are locked-in because if they move they lose the connections of both followers and those they follow, which often means family and friends.

Basically a solution similar to that adopted in Europe for phone numbers - that you can take your number with you when you move providers - would reduced social media companies down to "just a pipe for social media connectivity" which would ultimatelly kill those with the worst practices given that the barrier to entry to be a "social media provider" is way lower than to be a fixed line telephony provider.