What's to stop someone signing up with a fake email and sharing it on a forum so that many people can sign in with it? Yes, I know that there are ways to reduc

What's to stop someone signing up with a fake email and sharing it on a forum so that many people can sign in with it? Yes, I know that there are ways to reduce this by scanning IPs and so on, but by using third party auth you offload that onto the auth providers.