The problem was was that I had used a raw RSS URL for my feed
http://www.joombook.com/index.php?format=feed&type=rss
and this URL was in people’s news feeds.
With a new home page layout, that link will be broken.
My own advice that I failed to take is always to push a Joomla RSS url into a service like Feedburner. Then you can change the pushed URL as much as you like, then just edit it in (say) the Feedburner settings.
The messy solution in my case is to mess around in the innards of my htaccess file and create a redirect from the old raw URL to my new Feedburner URL:
RewriteCond %{THE_REQUEST} ^[A-Z]{3,9} /index.php?format=feed&type=rss(.*) HTTP/ RewriteRule ^index.php$ http://feeds.feedburner.com/TheJoomlaBook/ [R=301,L]
To make sure that there would not be any conflicts, I used the raw URL directly to the category in Feedburner:
http://www.joombook.com/index.php?option=com_content&view=category&layout=blog&id=7&format=feed&type=rss
Don’t make my mistake, listen to that clever Barrie North guy and Never Use Raw RSS URL’s in Joomla!