You need demand and user retention. Just the former is a false sense of success. Many startups flameout without the latter at high frequency. I've witnessed 100s.
(There are exceptions, of course. LASIK surgery, for example. In which case something like NPS can be a better key metric. But generally, repeat business is the best proof that you've got something real.)
Go.com w/ Overture, acquired by Yahoo - that had PPC offerings before Google even had a monetization strategy.
Sounds like a road map.
(Wikipedia)
And the better the market the more they convert (member conversion rate), and finally pay (customer conversion rate).
If you only measure conversions, you don't understand why they happened and what to improve to make them happen even more.