How does Facebook detect images when adding a link?

Question

When you add a link to your Facebook page, after some processing, Facebook presents you a next/prev button to choose an image linked to the url your are inserting.

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Answer 1

I am not familiar with the way Facebook did it, but I can tell you how I would do it.

1. Establish web request to desired url
2. Parse resulted web response with regular expression, that looks for