Hi,
please post me your site link, we will see the issue.

I think that your "template override" include the button into HTML form declaration, so when you click in the button, the form (with username and password) is submitted.

Hi,
I found the problem, Template Menu Javascript capture all click on top bar (and their child), so the click on our Facebook button not work.

to solve this try to add this line at the of file /plugins/system/t3/base-bs3/js/menu.js

jQuery(document).ready(function(){jQuery('li.dropdown.mega').unbind('click');});