I wonder if it's a referrer check. `if referrer == 'facebook': display_ads()`. That way the original viewer doesn't see the ad-filled page, but anyone clicking on the Facebook link does.

Yup, by default when you share a link using the official buttons it'll add tracking codes and referer query parameters to the request, easy to just slap on ads and crap if the link contains that.

I'm surprised FB hasn't handled this the same way Google penalizes you for hiding content from users that is present in for the bot.

If somebody is giving your users a 2nd-class experience, and you control the pipes, shouldn't you punish them?

The imgur method. Very annoying.