Googler Braden Kowitz:
Teams should focus on metrics that indicate:
Happiness – Often measured through a long-running survey that includes a Net Promoter Score.
Engagement – Visits per user per week. Or posts per user. In Gmail, we carefully watch a metric we call “5 of 7″. It’s the percentage of users that visit the product at least 5 out of 7 days a week. (This metric turns out to be predicative long term retention)
Adoption – Measured as daily active users (DAUs). Or commonly at Google: 7-day-actives, which is the number of unique users who have used the product once in the last week.
Retention – There are many ways to measure this. I like looking at the percentage of 7-day-actives who are still 7-day-actives a week later, a month later, and 3 months later.
Task success – This can be measured by looking at abandonment rates in any task, or looking at time-to-completion of key tasks. For example: on Quora, you may measure how long it takes to answer a question, and how many people start to answer, but do not finish.