I'm curious because there is a tab on the top of the screen for showing which of your friends are online and who has the game. This would be useful, but only my Xbox account is linked and I can't find any way to link Facebook as well, at least from my phone. How do you link more accounts to the app, if possible?
You need to link multiple authentication methods to your bungie.net account for this to happen. Everything you need is located [url=http://www.bungie.net/en/Clan/Post/18405/64336042/0/0]in this thread[/url]. Report back if you have issues.
You can link a Microsoft Account, a PSN account, a Facebook account, and a Google+ account to a single Bungie.net profile. I'm not sure if you can do this on the App, but you should be able to on the browser version of Bungie.net. Please read [url=http://www.bungie.net/en/Clan/Post/18405/64336042/0/0]this thread[/url] for details on this somewhat delicate process.