I can't even begin to fathom the magnitude of this considering how many people likely use the same login credentials for all of their sites.

The problem you run into is that communicating both the nature of the breach and convincing people to respond accordingly is incredibly hard.

This will continue to happen across many sites. I think after enough of these breaches, though, people will start to think about the protection of their online identities a lot differently, which is good, albeit at a painful cost.