This question evaluates a data scientist's product analytics and experimentation skills, including metric definition, causal inference, user segmentation, and measurement of engagement impact for a notification feature on a marketplace app.

Context: Circle is a US marketplace app for buying and selling second‑hand products. On a product listing page, a buyer can click “send message” to contact the seller. Each message sent counts as one listing interaction.
The team is considering (and then ships) a new feature on product listings:
How would you decide whether this is a good idea for the product? In your answer, cover:
The developers have shipped the functionality. How would you understand its impact and determine whether it is a successful feature?
Be specific about:
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