What’s the best way to measure product usage?

David Jackson (Founder, Seeking Alpha)
The best startup metric: Share of habit? | A Founder’s Notebook

Tom Tunguz argues that share of habit is a better metric for startups to focus on than engagement. Share of habit, however, has disadvantages. You can’t measure it if you don’t have access to accurate market size data and who has that? And its not a good operating metric, as its impacted by external factors out of your control. In that respect, its a vanity metric.