Content Locker (Facebook Share button)

Hello, I need a content locker which is "gone" only by "sharing" the page on fb (I saw something similar on FB POST LOCK, but its for wordpress) . In other words, when entering a site, a dialog will pop (which you cant get out of it) and will say "in order to see content you have to share" . When you share, dialog vanishes and user see the content that it hides. Thanks.