Fring allows video calls with skype who's users surely will outnumber those of the iphone.
Additionally, when someone buys an iPhone, they are not going to use that phone for the rest of their life. People who already have iPhones will buy new models. Others will migrate from iPhone to other phones. So just based on the number of phones sold, you can't tell when it will "catch up".
Furthermore, as Skype runs on the iPhone (over 3G as well now), iPhone users can also be Skype users.
You're right that you can't tell when the iPhone will catch up purely based on those numbers. I'd say it's a smart bet, though, that it won't be long. We know that Apple has already sold a few million iPhone 4s, and I'll bet that it sells somewhere in the neighborhood of 50 million before the next iPhone comes out.
You're also right that you can Skype on the iPhone. We don't yet know how good Skype video chat will be on the iPhone, how it will compare to FaceTime, or how many people will use it instead of FaceTime. We also don't know who else (if anyone) will use the FaceTime protocol.
However, we do know that every iPhone 4 comes with FaceTime enabled, that it is trivial to use, and that it is integrated in the phone and contacts at the system level. All of this softens the significance of Skype's apparent head start.