Your keystone metric is the most potent metric that drives growth, retention and virality in your product.
If Facebook gets you to add 10 friends in your first 7 days, then you more or less stay on Facebook forever.
If Twitter gets you to follow 30 people in your first 7 days, then you more or less stay on Twitter forever.
Keystones reflect density of social connection.
You don’t determine your keystone. It exposes itself upon use. Once identified, you can shape it.