Sarah Tavel, a venture capitalist and former PM at Pinterest, believes that growing and expanding user engagement is the key to building a valuable and lasting company.
The goal of the first level of engagement is to grow the number of engaged users completing your core product action. The core action is the primary reason people the product and is likely to be strongly correlated with user retention.
The second level of engagement is retaining those engaged users. This is done through accruing benefits and mounting losses for the user, meaning the more someone uses the product the greater value and reliance they have on it.
The third level of engagement occurs through virtuous loops within the product. Examples of this could be network effects or triggered notifications based on user action. These are typically difficult to create, but if executed well, can drive incredible product growth.