Fortunately, you don't have to do anything special - Playblazer already returns the "social" leaderboard as part of the "Get User Leaderboard" API call.

The "you" entry in the output of "Get User Leaderboard" API call contains both the global rank and the relative ranking among friends of the user.

The output also contains a list called "friends" containing sorted entries for friends - sorted by their ranks.
This list also contains the entry for the calling user - so that the app doesn't have to do any processing to add the user at relevant position in the list.