206 means partial content, which is what your Website/Server is delivering to Facebook's request. Have you tested the "Fetch as Googlebot" under Webmaster tools to see if Google can get the files? https://www.google.com/webmasters/tools/googlebot-fetch
If you get an error there, then it must be something IP related with your server, as my test returned a 200 and a test using Googlebot as the user agent also returned 200, which means that the IP wasn't blocked (nor the user agent excluded), basically telling me that if Googlebot is unable to access your site (nor facebook) it must be something IP related.
Hope that helps!