Growth hacking is finding fast growth by combining marketing, product, and data. Dropbox: "Invite a friend, get 500MB free" — a referral loop that grew users from 100K to 4M in 15 months. Airbnb: automatic posting to Craigslist — using existing traffic. PayPal: $10 for a new user — creating a viral loop.
The growth hacking funnel: Acquire → Activate → Retain → Revenue → Refer (AARRR). There's a growth opportunity at every stage. Virality coefficient K = i × c (i = friends invited per user, c = their signup rate). K > 1 means exponential growth. Experiment velocity — how many tests you run per week — is the core metric of a growth team.
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