I have a page with OpenGraph meta tags and when I first published the page it worked like a charm:
Now, after a few weeks, somehow my post does not show the image at all. (it shows a gray rectangle, though.)
The Facebook debug tool, here:
does show the image correctly.
This happens with many of my Wordpress & Drupal posts, which is problematic.
Remember what I said: at first it works. It's only after a while when I go back to my page that somehow the image is gone.
An example of such a post is this one (although I reposted in an attempt to fix the problem, but nothing got fixed, it's still missing the picture.)
If it were just one post, I would ignore the problem, but now it's been like 10 posts that lost their image.
Important Note: I do my Facebook posting manually as I know that using a tool, or even just AddThis.com can create problems to the posts. That's also how I know that it worked the first time I posted. I also first use the debug too mentioned above to make sure I get a thumbnail.
As a side note: I noticed a similar problem on Twitter, but I've noticed that with many website so I'm less worried about that one at the moment.