In 2009, Twitter was everywhere. Millions of people signed up. They landed on a blank screen that asked "What's happening?" and figured they didn't have anything to say. New users didn't stick around because they didn't understand the product. So Twitter rebuilt the onboarding flow to teach them each core concept one at a time. The redesign moved retention more than anything else they shipped that year. a16z's @joshelman on why onboarding is where you tell your product's story:…