Substack · Metrics · Mid · 45 to 60 minutes

Which discovery channel should Substack back?

Substack is a platform where writers publish newsletters and other posts, build audiences, and earn money from subscriptions. Answer this product management case and get it graded line by line against a weighted rubric.

Metric design
Attribution
Prioritization
Experiment design
Product communication

The situation

Your VP of Product asks you to prepare a recommendation for next quarter's growth planning. Substack has several ways readers discover and subscribe to writers, including creator to creator recommendations, web entry points, and multiple in app discovery surfaces. Creator demand for clearer acquisition reporting is rising, but internally the team does not have one trusted view of which channel is strongest, which one is accelerating, or which parts of the app deserve more investment. Assume the two largest channels each appear to drive roughly 2 to 3 million subscription starts in a recent month, and the current gap between them may be inside normal measurement error. The question is not only where subscriptions come from today. It is where Substack should invest next to help creators grow without becoming overly dependent on outside platforms that may suppress outbound links, and without making discovery feel noisy or low trust. You need to recommend how to compare channels fairly, what metrics should be shown to creators, and where product effort should go next quarter despite imperfect attribution.

What to produce

Write a 1 to 2 page product memo, about 700 to 1000 words.

Your answer must

Define a clear channel taxonomy and the decision you are making, including what counts as a subscription acquisition source.
Propose a metric framework with one primary metric and 3 to 5 supporting metrics that let Substack compare discovery channels over time.
Identify the biggest attribution and measurement risks, and specify what additional instrumentation or reporting cuts are needed to separate app discovery, recommendations, and web sources.
Recommend how to prioritize next quarter's investment across at least two channel families, with the expected upside, main trade offs, and what you would not do yet.
Describe a creator facing reporting view that exposes where subscriptions come from, including what to show, how often to update it, and how to explain uncertainty or mixed attribution.

Constraints

Do not assume Substack can control the policies of external social platforms or rely on them for predictable distribution.
Preserve creator and reader trust, avoid plans that depend on spammy growth loops or misleading attribution.
Keep the proposal feasible for the next quarter, with modest product and measurement changes rather than a full discovery system redesign.

Format

Use short sections: decision, metrics, measurement gaps, recommendation, creator reporting, risks and follow ups.

How Substack actually played it

Substack states that its app has overtaken Recommendations as the top source of subscriptions, with the app driving about 3 million in a month against 2 million for Recommendations. It attributes most app growth to Notes, the Home feed and onboarding suggestions. A case on how a platform shifts its main acquisition channel over time and exposes the numbers to creators.

Re-rank your growth sources regularly because the leader changes.
Attribute growth to specific surfaces (Notes, feed, onboarding) to decide where to invest.
Expose the metric to customers in a dashboard.
Compare your discovery against social platforms that penalise outbound links.

Built from The Substack app is now the most powerful growth machine for creators by Hamish McKenzie, Substack.