Mobile users are directed to an "external link" that opens in the Facebook browser. When you use Facebook sharing or Facebook login options, dialogue windows should open. Unfortunately, since 2016, Facebook no longer allows the opening of dialogue windows from the Facebook in-app browser.
For instance, when clicking on the "Share on my wall" button in this campaign:
Normally, this "Share on my wall" button should open a dialogue window such as this one:
However, since dialogue windows are blocked on the Facebook in-app browser, nothing happens when one clicks on "Share on my wall".
Get the Facebook dialogue to work by checking the dedicated option.
Go to your campaign's Settings step and click the Enable Facebook in-app browser compatibility setting. This will resolve the issue by adding an overlay with a "Participate" button:
This button will then open your campaign out of the iframe ("_top" target) so that participants can use Facebook sharing and login buttons: