1.) Click on http://www.facebook.com/editnotes.php?import
2) Add http://www.sheldoncomics.com/index.xml as the feed
3) It will ask to confirm the settings.
4) It will show the comic in the "My Notes" section of "Notes". (Facebook's algorithm shrinks the comic to fit the page, but a simple click shows the full-size version from the site.)
Try it out, and let me know how it goes.
(Note: I think it might also be possible for me to set up a one-click button using this URL: http://www.facebook.com/share.php?u=http://www.sheldoncomics.com/index.xml
...I'll test it out, and if it works, I'll add that button to the list on the left.)