Facebook removed the image previews for all links to altCensored.com, a community catalog of removed or limited-state videos on YouTube. This came about a week after the company prevented users from sharing links to Brighteon.com and NaturalNews. These moves to prevent or restrict link-sharing are likely due to the fact that these sites host content that Facebook works overtime to bury.
From Reclaim The Net:
… for links that were posted to altCensored after May 28, Facebook has started blocking the preview image when they’re shared on Facebook. Pasting these links into Facebook’s sharing debugger tool confirms that the image previews are being blocked by Facebook.