Facebook's active user base surpassed 1 billion in 2012. It's easy to look at the massive growth of Facebook and see it as a sort of big bang effect—a natural event difficult to pick apart for its separate catalysts. But Facebook's growth can be pinned down to several key strategies.

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Again and again, Facebook carved out growth by maintaining a steely focus on user behavior data. They've identified markers of user success and used those markers as North Star metrics to guide their product decisions.

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Facebook used analytics to compare cohorts of users—those who were still engaged in the site and those who'd left shortly after signing up. They found that the clearest indicator of retention whether or not users connected with 7 friends within 10 days.

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Once Facebook had identified their activation metric, they crafted the onboarding experience to nudge users up to the magic number.

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By focusing on a metric that correlates with stickiness, your team can take a scientific approach to growing engagement and retention, and measuring its progress.

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