Weird. Here's
all the ways advertisers can target Facebook users, so there must be something that's tipped them off to your purchase. Image recognition is a really interesting idea, though I think it would be pretty big news if Facebook actually found a way to implement that at their scale.
You're using AWStats? The
Direct Address / Bookmark / Link in email... section is actually a line item of its own, separate from the sites that are sending referral information. Since AWStats is generating its reports from your raw server logs, it shows requests to all of the files (including images) on your website, not just the pages themselves. When one of your images embedded on another web page is requested, the browser will send along the address of the page that is requesting it to your server. This "referring" page will show up in AWStats, though it doesn't contain an actual hyperlink to one of your pages. This is known as "hot linking".
For more accurate statistics about who's really visiting the pages of your site (and where they're really coming from), use a JavaScript-based analytics service such as
SiteMeter or
Google Analytics.