Post to Facebook - URL

If your protected content redirects to your Login page for non-logged in users, that behavior is expected initially. When you first add the Like button to your site, Facebook tries to scan all content periodically to see what's on your page(s). If that page redirects to the Login page, that's what Facebook will see. There's no immediate way around this.

However, as Facebook sees more of your public content *and* more registered user's visit your private content, Facebook will start using the information about your registered pages for the Like buttons. Since they can't scan in the background though, it will take longer for them to 'see' the pages and know what's on them.

If you just added the Like button to your site, something you may want to do is:
* Drop the registration requirement for your pages, temporarily
* Use the URL Debug tool to scan as many pages as you can. That will update Facebook's cache with the proper data and let them understand more about your page (mainly, that it isn't malware/bad)
* Re-enable the registration requirement

Hope that helps give you some ideas,
Alex