Make a URL shortener service which allows you to send a link to someone from within facebook - but it blocks all known FB domains and IPs from viewing said URL.
It will only take the first click on it then expire. if all the first clicks are from FB - it would mean that FB is pre-emptively scanning and tracking all URL messages - even before users actually visit.
Facebook certainly does pull any pages you link to, it scans 'em for images and puts those images into the message too along with a title and excerpt. This is occasionally annoying but makes messages prettier.
Make a URL shortener service which allows you to send a link to someone from within facebook - but it blocks all known FB domains and IPs from viewing said URL.
It will only take the first click on it then expire. if all the first clicks are from FB - it would mean that FB is pre-emptively scanning and tracking all URL messages - even before users actually visit.