I've run across a FBML compatibility issue with the AddThis plugin for WordPress.
The namespace that is added to the HTML tag via the language_attributes() filter/hook is out of date. It should reference the new FB XML namspace of http://ogp.me/ns/fb# as http://www.facebook.com/2008/fbml no longer exists. This is causing other on-page FB apps to fall over.
Any ideas people?
You can edit the plugin to change it, and I've let our developers know that we need to change it.
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