> What are they doing? Scale is harder to manage as the userbase grows. A lot of popular services could run on one machine if you can live with 3 nines of reli

> What are they doing? Scale is harder to manage as the userbase grows. A lot of popular services could run on one machine if you can live with 3 nines of reliability and never scale past 100,000 users. When you reach that scale, you also start doing lots of product work that's either exploratory and unlikely to matter, or micro optimizations trying to move the needle 1%. If you hit a win on one of these, because of your scale, the win can be huge.