Here is how I would answer the question (Feedback welcome) - You are the PM for Facebook Groups, determine the north star metric or goal for the team.
Let me start by going over the framework I would use. 1) I will define the goal of the product, 2) understand key users and the actions that are must for the success of the product, 3) I will define metrics that align to those actions and lastly 4) recommend a overall north star metric for the product, discuss drawbacks and tradeoffs. Does that make sense?
Goal: FB mission is to build communities and bring people together. With that in mind, FB groups provide a great opportunities for people who have similar interests to get together and connect in a safe space. So I would say goal of FB groups is to build communities and bring people with similar interests together.
There are two types of FB groups - the creators/moderators and the members.
The creators need is to create a group for members with similar interests. They can be experts in that area or may just be the first ones to think of creating the group. The key actions performed by these creators/moderators are:
a) Create a group
b) Add members
c) Moderate posts
d) Add and Host Events
The members goal is to participate in groups where they can ask and answer questions, comment on posts and learn from people who have either gone through what they are going through or help others who might need help. The key actions that they perform are:
a) Aware of a group or Search for a group
b) Become part of a group
c) Add a post to the group
d) Comment on group posts
e) Participate in group events, etc.
While thinking of metrics for these key actions, the most important area that would make Groups successful is if members are actively participating in groups. So, I would focus on engagement and retention goals. Some key metrics could be:
a) Number of customers with at least one group - penentrations/ awareness.
Then I would like to talk about usage in general
b) Number of groups created daily, weekly
c) Number of members added to a group daily, weekly
d) Number of new posts/ comments in groups daily, weekly
e) Number of total interactions in groups - including new posts, comments, reactions, etc.
f) Number of group events created, participation, RSVPs.
Lastly, would like to focus on retention
g) Groups DAU/MAU - how many customers come back to participate,
h) First time users DAU/MAU.
While prioritizing and thinking about the north star metric, I think the more FB users actively participate in the group, the higher connections they will be able to make with people with similar interests - which was our goal. Hence I would pick Groups DAU/MAU as the north star metric. I like this one also because it includes new acquisitions in the DAU as well as DAU will be impacted if we have new groups getting created and more members posting/commenting in groups.
Couple things I would consider as secondary metrics for tracking and troubleshooting - may be more tactical are - number of posts/ comments daily - which kinda shows how many interactions per daily active customer.