The language attached to freemium business models requires specificity because these businesses associate expectations with distinct user groups. With freemium, there’s a vast canyon between free access (users) and pay access (customers); they are not synonymous. That’s why the following clarifications are necessary:
Customer — A converted user who now pays for premium access or services. Example: As my storage needs increase and I become more reliant on Dropbox, I’ll likely convert into a paying customer.
I realize this entire post teeters on nitpicky semantics, but heated debates require clear boundaries.
Sidenote: I highly recommend the Times’ Evernote story. It’s a great representation of the opportunities and obstacles that come with freemium models, and it has actual numbers.