Although the terse description tells you to "Enter a URL to see some helpful feedback about your page markup," the tool actually pulls the info for you as well as debugging the link to pull the correct title, image and description when you put the link in a Facebook post or comment. This will also repair the links when they're posted in scheduling services like Hootsuite.
I discovered this handy tool when Facebook kept pulling old titles and descriptions for one of my clients' websites. I was pulling my hair out trying to update the links - working with the web guys to update the page's back end, checking and re-checking it, but to no avail. Facebook and Hootsuite both kept pulling the old cached version. Thanks to the Debugger, the pages are all updated and I still have all of my hair.
If you have any questions on debugging links for Facebook, or any social media management questions at all, I'd love to hear from you in the comments or via email.
Happy posting!
