Lena Nitsche 
Hey everyone. Question: Since a couple of days Facebook isn't showing the thumbnail pictures of posts I want to share on the GV German Facbeook account.
Any tips on how to let the thumbnail pictures appear again? Thanks!
This is a great question, and one that comes up a lot, so this post will explain why it happens and how you can fix it with the Facebook Sharing Debugger  tool.
WAIT! What about Twitter? Twitter now has a similar tool they call Twitter Card Validator ! It works just like the Facebook Debugger as described below, so if you have post with an updated title/excerpt/featured-image, consider going to both places and “rescraping”/”Previewing” the data to reset all existing shares!
What is going on with the sharing preview?
The first time a URL is shared on Facebook they send a bot to visit the URL and check the post metadata which is embedded in the code. The metadata it needs are things like the URL of the Featured Image, the title, the excerpt and the author's name. This data is always available on Global Voices posts, if the post is visible, the data is in the source code.
When Facebook finds this data it saves a copy (cache) and holds on to it forever. This is theoretically good, because when new people share the URL it can happen faster, but there's a problem: If the bot can't load the post, it saves empty data and never updates it!
The effect is that any error with the page during that first share will make the post broken forever unless you manually reset the cache with the Facebook Sharing Debugger  tool.
There are a variety of reasons why a post might be “broken” when Facebook first visits it. Some are Facebook's fault, some are ours and some are just weird . Regardless of the cause, when it happens it's vital that we fix it ASAP, since Facebook will never try even one more time to get the correct metadata.
Resetting the cache
Luckily it's easy to fix a post's sharing metadata using the Facebook Sharing Debugger  tool.
- Visit Facebook Sharing Debugger 
- Paste in the URL of the post
- Click Debug to load the post info
- Click Scrape Again to have it rescan the URL
- Check that all the data is right and the image is showing.
That's it! You should now be able to share the post on Facebook and have it look great like normal.
Note: The debugger has it's own bugs. Sometimes after clicking Scrape Again you have to reload the page, or scrape yet another time, before the correct image shows. In my experience, the data is correctly loaded after the first rescan, but the preview doesn't show it right away. Just something to keep in mind in case it makes you feel crazy :)
This tool also has a lot of other uses for developers (in GV's case, that's me) that you don't have to worry about. Just remember that the Facebook Sharing Debugger  exists and can quickly fix any posts you notice are broken.
Final Thought: Always test Facebook sharing after publishing a post!
A related rule we should try to follow is to make sure we share each new post to Facebook at least once after it's published. This is a good idea anyway, since we want to promote our work, but also ensures that there are no problems with the metadata.
If you share a post to FB right away and it looks right, then you know this problem will never happen for that post, and if there is a problem, you can fix it before any users share the post.
Thanks as always for your attention, hope this was helpful and please let me know if you have any questions!
The Twitter “Card Validator” does the same thing for tweets!
Twitter now has a similar tool they call Twitter Card Validator ! It works just like the Facebook Debugger as described below, so if you have post with an updated title/excerpt/featured-image, consider going to both places and “rescraping”/”Previewing” the data to reset all existing shares!
I noticed that I had to “Preview” a URL twice in the twitter tool to really refresh how my tweet looked, so I recommend pushing the button twice and checking an existing share of the URL to make sure the changes are applied.