When someone shares a SharePoint public facing website via Facebook, Facebook loads a “preview” of the website in the post. What it does is gets the URL and grabs the first image it finds that SharePoint is presenting to the world.
The problem is, unless overridden, the first image it will (often) choose is the SharePoint icons sprite that looks like this…
Ick. That won’t look good.
The Fix: Put in some code in the master page in the HTML head:
<meta property="og:image" content="URL OF DESIRED GRAPHIC – company logo maybe" />