What metric to measure product managers?

We're building a recruiting and employee management platform (similar to Yammer).

The product records and analyzes metrics for each different type of employee so that we can offer employers of large organizations an overhead view of how their team is doing.

What metrics would you say are important to track for product managers?

Metrics that we could use to decide how productive a product manager is or how important their contributions have been to the team.

The most useful metric for product managers is engagement of users on the products they launch/built/maintain, number of users is secondary, and everything else isn't as good. If you only need internal metrics and company is on Agile, than you could use point value of stories PMs write that get completed, difference between points estimated and actual, size of the backlog, etc.

Ideally, Product Managers should have a P&L (revenue) responsibility for their products - a difference between projected and actual is how performance should be rated.

EDIT: Just wanted to point out that I personally wouldn't recommend using points to measure performance of any team member, just wanted to offer an option based on data easily available (not the same as most accurate). Thanks to rbwhitaker for the nudge to correct my POV.