As far as milestones go, 200 million is a pretty big number. Pandora (NYSE:P) announced Tuesady morning that it has surpassed that mark in registered users -- an impressive tally, but one that needs to be put it in its proper perspective.

For starters, there's a big difference between 200 million registrations and 200 million users. Pandora puts out monthly user metrics, and we know that it had 69.5 million -- a little more than a third of its total registrations -- check in as active listeners as of the end of March. That's a big number, and it's up a healthy 36% increase over the past year. But it's not 200 million.

Investors also need to consider the value of these 200 million cumulative registrations. How many of them are duplicate accounts? How many of them no longer exist? What's a Pandora account worth?

We may never get an answer to the first two questions, but we can give the third query a shot.

Since Pandora's market cap is $2.2 billion, we can divide that figure by 200 million to arrive at a value of $11 per registered user -- or closer to $32 for each of Pandora's nearly 70 million active listeners.

The market's perceived value of Pandora's membership is low because the company just isn't good at milking revenue out of its users. Premium-radio leader Sirius XM Radio (NASDAQ:SIRI) has just 23.9 million subscribers, but it commands a roughly $20 billion market cap.

How can the average Sirius XM subscriber be worth $836 while the typical Pandora user can be swapped for a multiplex movie ticket? Well, Sirius XM consumers are paying good money for premium content. Pandora is still a haven for freeloaders. Sirius XM cranked out $3.4 billion in revenue last year, while Pandora rang up only $427 million on a much larger base of users, and it failed to follow Sirius XM into profitability.

That doesn't mean you can't make money as an ad-supported platform. LinkedIn (NYSE:LNKD.DL) also crossed the 200 million mark earlier this year, and most users of the career-oriented social-networking website approach it as a free destination. However, it generated more than twice as much revenue as Pandora did last year, and its market cap is actually closer to Sirius XM's these days.

In other words, Pandora has a long way to go.

Instead of praising the 200 million-subscription milestone, maybe Pandora should be taken to task for how little it has been able to do with such a wide audience. Pandora generated just $51.9 million in subscription revenue last year, and that's a little more than $0.25 per account over the course of an entire year if we go by that cumulative registration figure that the company's shouting from the rooftops.

Pandora's posting impressive growth metrics, but now it's going to have to start growing where it counts.

Longtime Fool contributor Rick Munarriz has no position in any stocks mentioned. The Motley Fool recommends and owns shares of LinkedIn. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.