In the coming days Facebook is planning to give publishers an option to lock Instant Articles shared on the social media service and charge for the full article access.It would be a great opportunity for the publishers who use Facebook to share their content. But the social giant may face some problems while implementing the paywall.Econsultancy’s Patricio Robles has shared six challenges that Facebook might face.Robles says, “Here are six of the biggest that publishers should keep in mind as Facebook gears up to test the subs model.Developing a workable subscription structure... [...]