Unable to get the current user's friends that use the app facebook graph

So I'm not using facebook's SDK or anything. I'm making simple http requests.

When I do an HTTP request to /me/friends I get an empty array response. data: []

The current user has a friend that also uses the app, the current user actually has lots of friends that use the app. The current_user is an admin of the app, and the app is live.

I've also tried asking for user_friends permission after that, I get the following result.

{
  "data": [

  ],
  "summary": {
    "total_count": 536
  }
}

So still not getting what the friends who have also installed the app. Do they need to enable something? I've went into the apps I've installed and made it so others can see that I use the app. Does every friend also need to do this?

If anyone else ran into this problem, it'd be great to know possible solutions!