How to choose the metrics for measuring community engagement

We measured community growth only by counting member signups for the first six months. The numbers seemed good on paper. The discussion boards were quiet.

We changed our focus to a metric: the ratio of member-created posts to staff-created posts. We also watched how many questions got an answer from another community member within two hours.

The quality of community interactions improved a lot once we stopped chasing vanity registration numbers. Engagement became self-sustaining because community members actually helped each other.

Measure community health by peer-, to-peer interactions of raw headcounts that hide low activity levels.

What specific metric tells you whether community is genuinely thriving or just collecting accounts?