facebook login only with one domain possible

There isn't a way to have Facebook logins work across multiple domains. That's a requirement by Facebook and enforced by them. There's no way around it.

The 'Domain' setting for your application can be used to set multiple sub-domains, but not different domains altogether.

Hope that helps explain. If you have different sites with different domains, they'll each need to have their own application. If different domains all direct to the same site, then I'd recommend setting up a real redirect that actually switches the URL in the address bar to the domain configured in your FB Application.

Alex