Note: The question is not how to fix the problem, as that is documented elsewhere on SO (e.g., Integrating Facebook to the leads to blank pages on some browsers / fb_xd_fragment).
1) What causes this, and under what conditions is it triggered?
2) More importantly, does this affect end users at all? For instance, how does this bug affect the URL shared by someone who clicks the FB Like button? If someone clicks the FB Like button from URL A, does URL A still get shared (but with “fb_xd_fragment” appended), or does URL A become your root URL (with “fb_xd_fragment”)? In our logs, all the URLs appear as the root URL with “fb_xd_fragment” appended, so we’re not sure if this is because people are clicking the Like button from the home page, or if all the shared URLs get morphed into the root URL.
Basically, what happens is whenever you use the JS API it opens your site in another iframe to use as a cross-domain receiver. What you can do is set a custom channel URL and it will use that instead. If seeing this bothers you, you can set a custom channel url. More information on http://developers.facebook.com/docs/reference/javascript/FB.init/