Since it is not using the Facebook platform or API, what is Facebook's recourse? If it was using the API and there was a developer with a Facebook account they could "hold accountable" then they could do something, but there is no specific Facebook account associated with creating this software. Ban/censor the Gizmodo page on Facebook.com? That would be an interesting gambit that would surely backfire in the court of public opinion.

This is just like Facebook banning anything which tries to track unfriending, another thing you can't do through the platform API and that they have actively worked to oppose and suppress.

Let's take Facebook's argument to its logical conclusion which would suggest that they should go on the warpath to ban all password manager applications that store a user's Facebook username and password.