This came up in conversation, but I'm sure the idea has been expressed elsewhere so it's probably credited to someone else.
However:
"Wright's Law: the number of users on any platform is inversely proportional to how ethical that platform is."
Conversation here:
https://mathstodon.xyz/@ColinTheMathmo/117060393923666917
Added in edit:
There is already a "Wright's Law" that talks about manufacturing, so maybe this is:
"Wright's Law of Social Media".
But I still hesitate to claim such an obvious principle that everyone will know, even if they haven't articulated it.