Important Conversion Metrics You Should Watch

How you define that will be application dependent but I always aim to be fairly stringent.

For a forum, something like, "Number of users that make at least 3 posts per week that are longer than 100 words" would be a good one.

Watching that number can give you an outstanding early warning system, alerting you to problems that might not be immediately obvious.

As soon as your most enthusiastic users stop being quite so enthusiastic you need to know.