What is the "Aha" moment of your product? How do you define it?

I was watching a video by Chamath Palihapitiya, he was talking about the early days on how he grew Facebook. He mentioned that getting user to the "Aha" moment as early as possible is something that they focused on tirelessly. Their success metrics was "A user should invite 10 friends on the first 7 days of them signing-up" What is your Aha moment? How do you quantify them?