Build the home timeline. A user opens the app and sees a reverse-chronological (or ranked) feed of tweets from accounts they follow. The hard problem is *not* posting — it's that 300M users each have a unique feed, the average follow graph is fat-tailed (Obama follows back nobody, but is followed by 100M), and reads outnumber writes 1000:1. Every design choice here is really a choice about where in the system you pay the fanout cost.