I use social plugin for Facebook comments. Nothing unusual initially. However after migrating to new hosting provider, I started to see the tiny Facebook warning under the comment box. It say something about the URL being unreachable.
I tried few things, to the extend that I change to another hosting provider for this sake (because I read somewhere that might be due to the server blocking robots from Facebook from fetching the content).
I never thought that it was due to the meta content that Facebook is looking for.
Just add og:title, og:image, og:type and og:url then the “unreachable” problem gone.
Hope this help those who are facing the same issue as this.