Hi Dan,
There's not really an easy or accurate way to measure, this, since web activity tracking is dependent on the persistance of a client-side resource, the cookie, which can A) be cleared by them and B)does not persist across multiple browsers or multiple machines. With that in mind, I'd say the best estimate you could get is probably to create a smart list like this:
Visited Web Page
Web Page is Any
Data of Activity in Past 90 Days
This will give you a count of the number of leads which visited a page and were known in the past ninety days and give at least a reasonable estimate of how many leads are currently being tracked as known.