question

The solution for showing mutual friends/ followers is to set the following player's read only data to contain the other player's id. how would this handle facebook mutual friends?

Currently, PlayFab can't handle Facebook mutual friends. Since PlayFab has already integrated with Facebook for their friends lists and Facebook friends will be automatically returned in the response of API GetFriendsList. So that PlayFab can't tell whether the Facebook friends in the friends list are manually added on the PlayFab side or automatically added by Facebook side. So, there's no way to tell which are Facebook's mutual friends, you may need to complete this function through the friend list fetched from Facebook SDK.

basically, the friend's list has already been setup. do i need to manually check everytime the friend's list gets updated?

Yes, the friends list won’t update automatically, you need to call API GetFriendsList to update the friend list.

also is the read only data scalable? what if i want to include profile pictures etc..? does that mean i have to api call every mutual friend?

You can store the followers’ PlayerIds and his ImageUrl in the player's read-only data together, and the value of the player's read-only data has a limit of 300,000 bytes. We recommend that you don't show the exact number of mutual friends in “Suggested Friends” so that you only need to call the API once.