Strange domain behaviour - Facebool returns wrong image

I have a handful of subdomains on a wp network and use the domain mapping plugin. It works fine but i am getting very strange behaviour on one domain. The subsite is verysimplesites.maddiemcmahon.com the mapped domain is maddiemcmahon.com. When I look at the site all is fine but try to paste a link on Facebook and the image that shows up is a domain holding page (you know the one - pretty smiling blonde girl) - the link works (as far as I know). When the url shortened verison of the post is used (ie using the subdomain address) the image is the correct one. All the other mapped domains show the correct image when pasted into Facebook.

There are no doubt a number of factors that could cause this to happen but on the off chance anyone here has any suggestions I thought I would ask.

Many thanks