I have a fb share button on my website (angularjs), and i am wondering whether, when a user A shares the page content on a user B's fb wall, it is possible to get the user B's fb id.
thanks, regis
Not possible at all. What´s on the wall of user B is none of your business if user B did not authorize your App. And you can´t get the ID of user B if he did not authorize your App. You would not be allowed to use that ID for anything anyway without his authorization, so it would be pointless to get it.
Also, it is not possible to know where something was shared with the share button. You can take a look at the share dialog though: https://developers.facebook.com/docs/sharing/reference/share-dialog (see the response value)