What is the best metric to measure your startup against?

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 it’s not a good operating metric, as it’s impacted by external factors out of your control. In that respect, it’s a vanity metric.

David Jackson in The best startup metric: Share of habit?