Yeah, I believe I fixed this on lastest NodeBB code.
[Solved] OG Image URL Broken
<meta property="og:image:url" content="http://community.nodebb.orghttp://community.nodebb.org/uploads/site-logo.png" />
This is not proper formatting.
Should be fixed now not sure why the site image had an absolute url.
I didn't know if this was intended or not, but I guess it was a bug.