Connect app users with a mutual friend

I'm looking to create an app that allows people to create connections with each other. I'd like to be able to recommend that somebody connects to another user who has authorised the app if they have a mutual Facebook friend. I do not need or really want to pull out any information about that mutual friend.

I see a lot of people here asking about pulling lists of friends of friends and how that isn't allowed as those people haven't authorised your app. This seems like a very sensible (and clearly deliberate) limitation set out by Facebook.

What I want to do is a little different. I want to pull specifically friends of friends who HAVE authorised the app. It would seem that if I have offline access for this, I can access the friends lists for all my users, just not take intersections of them. Am I right?

An example already doing something like this would be http://airbnb.com. I can go there, log in with Facebook, and then search for properties that are owned by people I share a mutual friend with. Must they be doing this by downloading the list of Facebook friends for each user, storing them in their database and then computing the intersection with the logged in user themselves? Giv开发者_JAVA技巧en that all the permissions are there, this seems like a rather roundabout way of doing things compared to a nice FQL query.

I guess the issue is that you can only use FQL with one access token at once. Computing mutual friends of two app users would require permissions from both of them and hence two access tokens?

Any insight anyone can provide about how to do this would be greatly appreciated. I don't think it would be using any data a user hasn't given permission for but I can't quite figure it out.

There is mutual friend API that you can use. Given two facebook id's it returns the list of mutual friends, so you don't need to do any of what you have written.