Whether your a blogger, social media marketer, or even just a small business owner, at some point you’ll probably run across the issue of Facebook not displaying the correct image from your WordPress site. Sometimes Facebook might pull an old image or even worse, no image at all! Thankfully though you can easily troubleshoot and fix these types of issues using a tool called the Facebook Debugger. Check out some common scenarios below that you might run into and how to fix them. How Facebook Fetches Information
Whenever you share things to WordPress, Facebook caches your images on their servers and CDN. You can think of this the same way caching works on your site, sometimes you have to clear the cache to get things working again. The same sometimes goes for Facebook. If you make updates on your site, Facebook might still show an old image when you go to share it, this is because it doesn’t fetch new information, but rather serves up the information it already has cached.
The same goes for information about your link and content itself. They fetch what they call Open Graph Meta Tags. According to Facebook, the Open Graph protocol enables any web page to become a rich object
Source: https://managewp.org/articles/16832/how-to-use-the-facebook-debugger-to-fix-wordpress-images
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