When most people ask about a business growing, what they really mean is growing revenue, not just growing the number of people using a service. Traditional businesses would view people using your service that you don’t make money from as a cost.

Facebook is inherently viral. There are lots of sites that include a contact importer, and for lots of them it doesn’t really make sense. For Facebook it fits so well. It wasn’t until a few years in that we started building some tools that made it easier to import friends to the site. That was a huge thing that spiked growth.