You’re seeing these in your user agent app logs and wondering what’s going on? Is this a link from facebook.com or some kind of sinister bot?
When someone posts a link to your page on facebook, you’ll notice a preview image of the page will often show up- so facebook is going out and loading your page url and scraping the content to figure out what’s on the page, and what should be shown for the preview… it also is just checking that the url is valid and not some bad/spammy/malicious content that should not be allowed. Since this is effectively a bot hitting your site, facebook sets the user agent to:
facebookexternalhit/1.1 (+http://www.facebook.com/externalhit_uatext.php)
Note that if your site is set to block bots, it may block this request and thus facebook won’t get a valid response to the request- so it may either disallow the post or it may allow the link but just not show any preview content for it.
Do you want a specific images to be shown on facebook when a link is posted? You can use OpenGraph headers to specify meta data to use for this, and set the og:image tag to point to the image you would prefer be used.
For more info, here are references for this content:
Meta Web Crawlers: https://developers.facebook.com/docs/sharing/webmasters/web-crawlers
Images in Link Shares: https://developers.facebook.com/docs/sharing/webmasters/images