(The steps below also apply if Facebook is showing the wrong link image thumbnail.)
Occasionally when sharing content on Facebook you’ll find that the link preview is broken like this:
Here are the steps to fix it:
1. Visit the Facebook debugger tool – https://developers.facebook.com/tools/debug/
2. Paste in the problem link and click the Debug button. The debugger will return the info it has on the URL and likely include an error.
3. Click the Scrape Again button.
The debugger should now return more information about your site, and display a working preview of the link too. Go back to Facebook and share your link again. The link preview should now work as expected.
Why does this happen?
Occasionally when Facebook visits a site to grab the preview, it fails to do so correctly. Facebook caches whatever info it can grab about the URL (temporarily stores the info in memory). The next time the link is shared, Facebook uses the cached info and an incomplete or broken preview is displayed. The steps above tell Facebook to delete the cached info and visit the site again.
There are many reasons why Facebook can fail to generate a preview correctly, the most common reason being a temporary connectivity error (the path between Facebook and the site was blocked), or the site was temporarily unavailable (plugin error, in the midst of applying updates, etc).