1) The document discusses metrics for measuring product usage, focusing on tracking the number of daily active core users who use the product frequently and regularly over time. 2) It defines core users as those who use the product directly without referral links, return to it weekly or monthly, and share it with others. 3) The author advocates cloning one's core users through word-of-mouth promotion and incentivizing users to bring their friends on board in order to increase virality and get more people to sign up and stick with regular usage over the long run.