Since July 2016, Facebook no longer allows the opening of a dialog window when clicking on a button within an iframe on a website displayed in Facebook in-app browser.
Normally, the “Share on my wall” button in your campaign’s exit screen should open a dialog window such as this one:
The problem is, those are blocked on Facebook in-app browser. So when you click on the “Share on my wall” Facebook button, nothing happens.
If you activate the Facebook in-app browser compatibility, it will resolve this issue so that when a visitor is on Facebook in-app browser, the iframe will be overlaid by the “Participation” button.
This button opens the campaign out of the iframe (_top). The participant can use any button, including Facebook Connect & Facebook Share.