Question

Using the Facebook API, you can get back the post ID of a Facebook share, even if the user has not authorized your app.

The post ID is prefixed by your user ID, and separated by an underscore.

FB.ui({
    method: 'feed',
    name: 'People Argue Just to Win',
    link: 'http://www.nytimes.com/2011/...',
}, function( response ){
    // Response gives you :
    // { post_id: "[fbUserId]_346759642059572" }
});

To me it looks like Facebook is using this programmatically, rather than with the idea of providing us the userId out of the kindness of their hearts. But it’s extremely tempting to use.

I’m a little rusty on permissions – if there is a way to get back all the users that have liked/shared a specific URL, and not just a count, then this should be okay.

But the question remains, is it acceptable to use?

EDIT:

Now that I think about it, you can access the user ID by making an anonymous call to https://graph.facebook.com/[postId] but ONLY if the post was made public.

Answer 1

You can only cache user data if you have permission or it is necessary for the function of your app. Using the like button you can subscribe to edge.create and cache the response from the like.

refer to: http://developers.facebook.com/docs/reference/javascript/FB.Event.subscribe/

II. Storing and Using Data You Receive From Us

You will only request the data you need to operate your application.
You may cache data you receive through use of the Facebook API in
order to improve your application’s user experience, but you should
try to keep the data up to date. This permission does not give you any
rights to such data.

in my app i store the id of every page or user that is viewed to speed up page load times and count page views. Seems with in the scope of Policy as long as i am not sharing the info with 3rd parties or making public what a user has set to non public.