Once a metric like this has been defined, it becomes much easier to understand what changes in the product drive it higher. Retention is usually the strongest lever. In the example of PayPal, if a user continues to send money over weeks and months and years, each transaction might help bring in new users onto their platform. In other words, their viral factor grows over time, slowly inching toward a magical > 1.0 metric. On the other hand, if users are always one-and-dones, then they have to invite a ton of users in one big spammy blast in order for the product to propagate—which isn’t ideal.
