If there is an error in your layout.master file it will load a failsafe layout.master to keep from crashing the site and it will log the error. You should look for errors in Administration > System log related to your layout.master
My guess is you made some mistake when pasting in that control and that made it load a different master page.
I can't think of anything about a skin or about that facebook control that would prevent login.
The most likely thing to break login is if you are using encrypted passwords and you don't restore your machine key in web.config, it can lock out users because the machine key is used as part of the encryption, so if machine key is changed by an upgrade it could lock out users.
Hope that helps,
Joe