Evernote has demonstrated that it can monetize its user base, and it has a strong lock-in effect (I store all my stuff there so it's not worth leaving to a competitor). I happily pay for Premium just like many others.

Instagram, on the other hand, has a fantastic free app, a giant user base...and no revenue.

In theory, that should be the same as saying 'my company is worth y% of what company Z is worth, so this is how much cash you need to pay in order to get x% of it', but in practice, you can't always make that conclusion based on how people behave.