Xkcd is one of my favorite comics, so sharing a good strip is part of my “sharing agenda”. Facebook has traditionally a problem with xkcd.com pages.
To be more exact, it just doesn’t find the image. Not only that, even when you give it the actual URL of the strip, it will refuse to show it. Here is the end result.

So every time I wanted to share a xkcd comic I ended up saving it locally and uploading it as a wall photo. Here is sharing the same URL on Google+
Success!
Google+ wins :)