Really interesting essay on the nature of growing a startup. What's staggering is how much of a difference a single percent can make. A startup that grows at 5% a week from a base of 50 users will, after two years, have 8,000. 6% will take you to 20k and 10% will take you to a million.
Key point is that focusing on growth—which is the key metric of any startup—turns it into an optimisation problem, which all decisions can then be based upon. If you're not growing, you're a side project, not a startup.
Of course, focusing on the wrong metric can be devastating. User signups are a vanity metric, so you should never focus on raw numbers. But engaged users or paying customers—that's a great metric to focus on.
All PG's essays are great, but this one is amongst the best.