Early on, after Facebook launched its Feed feature, the product team at Facebook was trying to understand how to engage and retain users. What the product team noticed is that IF a user got to 10 or more friends in 14 days, there was very high likelihood of this user sticking around for a while.
The team had a hypothesis that 10 friends collectively generated enough interesting activity to populate the user’s feed and to keep him or her coming back. For Facebook, 10 friends has literally become a magic number, because, while the exact mechanism behind it wasn’t clear, users with 10 or more friends after 14 days didn’t churn.
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