Using the Facebook Share Dialog provided by the Facebook Developer Page, I discovered it was not working in IE8. The error console provided an ‘invalid argument’ error. After some debugging I discovered it was the ‘name’ parameter of the window.open() call. IE8 fails to run the script if any dashes are used in the name parameter. Other browsers do not seem to have a problem with this.

The provided FB share code:

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<a href="#" 
  onclick="
    window.open(
      'https://www.facebook.com/sharer/sharer.php?u='+encodeURIComponent(location.href), 
      'facebook-share-dialog', 
      'width=626,height=436'); 
    return false;">
  Share on Facebook
</a>

The name parameter ‘facebook-share-dialog’ must have the dashes removed:

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<a href="#" 
  onclick="
    window.open(
      'https://www.facebook.com/sharer/sharer.php?u='+encodeURIComponent(location.href), 
      'facebook_share_dialog', 
      'width=626,height=436'); 
    return false;">
  Share on Facebook
</a>

Voila the share button now works in IE8.