Thanks a bunch for reading my post David :).
I think the performance metrics and expectations would be very similar to that of a growth hacker; acquisition & above all user engagement. I’m a firm believer of quality > quantity (for most social products) and especially for Islands since it’s all about the added value of the conversation and not about the amount of people that are in the group.
A psychological approach towards driving user-engagement would be the most useful and promising focus of a PIR . Think about psychologically rooted books such as hooked for an example of the successes that psychological insights can have on user engagement.