You're evaluating launching a Venmo‑like peer‑to‑peer payments feature in a large messaging app. 1) State the single most important business goal (objective function) and 2–3 concrete success metrics (precise definitions/units) for the first 90 days; include at least one guardrail protecting core messaging health. 2) Propose two monetization approaches (e.g., per‑transaction fee vs interchange/float) and how you would measure causal impact on ARPU without subsidy‑driven volume inflation. 3) Design a beta rollout that handles network interference and leakage: specify the randomization unit (user, dyad, ego‑network, or geo), exposure definition, how you’ll enforce access when a non‑assigned friend wants the feature, and how you’ll estimate ITT vs TOT. 4) If an RCT is infeasible, write the exact difference‑in‑differences specification (define treatment/control, pre/post windows, the estimand, required parallel‑trends checks, and clustering of SEs) and name two falsification tests. 5) List the top three risks (product, fraud/regulatory, ecosystem) with a measurable leading indicator and a mitigation you’d ship before full launch.