Communicating with the parent website

There are tons of apps in Facebook which are nothing but some embedded external websites. Let's take an app for discussion. "Who views your p开发者_运维问答rofile" or something similar to that app.

How can that app know? How they communicate with the parent website (Facebook) where they are embedded in? Or it's something like they get the people who show up in the profile page (Made comments/Liked a comment, pic..etc) and randomly generates a list of friends?

Is it possible to implement such app? If so, how would you communicate?

You need to use the Facebook Developers Program -- that is how these sites gather such data. The fact that it appears to be "a website within a website" is because thats how Facebook makes money -- ads, sending people to another site wouldn't generate any ad views for them.