Facebook is really into crawling the web these days and preparing the ground for a big fight with Google one day.
Below you will find a piece of evidence of how Facebook is indexing the external data.
Why does Facebook appear in my server logs?
Facebook allows its users to send links to interesting web content to other Facebook users. Part of how this works on the Facebook system involves the temporary display of certain images or details related to the web content, such as the title of the web page or the embed tag of a video. Our system retrieves this information only after a user provides us with a link. You may have found this page because a Facebook user sent a link from your website to other Facebook users. If you have any questions or concerns about any links or content sent by one of our users, please contact us at email@example.com.
Short link: http://➸.ws/~Dtm1$3w