It’s as inevitable as AT&T losing its exclusive grip on the iPhone: Growth in the wireless industry is slowing down – when it comes to new customers, that is. AT&T added just 400,000 new postpaid customers in the fourth quarter – less than half the 841,000 new postpaid customers it added last year – and the number of new postpaid customers flocking to Verizon Wireless dropped 30 percent, to 803,000 from 1.16 million last year. Sure, there were some extenuating circumstances last quarter as potential customers waited to see what would happen with the iPhone, but the trend of slowing subscriber growth isn’t a fluke.

Carriers are seeing fewer new customers signing up for service, and for good reason: There are fewer new customers to be had. More than 90 percent of U.S. residents already have cell phones, according to CTIA estimates. Verizon executives say they expect saturation rates to hit 300 percent some day, but current trends indicate operators are going to get to that number at a much slower pace than the industry has seen so far.

On top of slowing customer growth, operators are also seeing big shifts in voice and data ARPU. Data usage is driving up revenue as voice sales decline: Verizon’s voice ARPU slipped nearly 7 percent in the fourth quarter even as data ARPU rose 20 percent.

The big question, of course, is how much we need to be worried about this. On the one hand, there’s slowing growth and declining voice ARPU. On the other hand, there’s the likelihood that saturation rates will rise well above 100 percent and data usage will continue to drive sales thanks to connected devices.

There are other factors to consider as well. We won’t know how many of the new customers at AT&T and Verizon came from Sprint, T-Mobile USA, U.S. Cellular and other regional operators – there’s a chance they could see their customer base shrink as customers leave for the big two. We also don’t know how much revenue operators can bring in from connected devices over the long run – are we talking about substantial new revenue streams or incremental additions to profits?

One thing we can be assured of is that wireless operators be busy finding new ways of convincing their customers to pay more for connectivity, from tiered data plans to the extra $20 Verizon is charging its iPhone customers to turn on the device’s mobile hot spot feature. They’ll get their feature phone subscribers to upgrade to smartphones, take advantage of the growing demand for tablets and squeeze out incremental revenue from the M2M market, as AT&T is doing with the 1.5 million connected devices it added to its network last quarter. Operators might even branch out into other types of services to keep the momentum going, as Verizon is considering with its connected home trials.

It’s important to keep in mind that growth is more than the number of new customers operators can keep coming through their doors. It’s also the amount of data subscribers are consuming, the type of handsets they’re using and the amount they’re willing to pay for instant, always-accessible connectivity. Investors seem to get this message, since the net add declines at AT&T and Verizon didn’t have much effect on their stock. AT&T’s shares only dipped slightly when it said the quarter would be “rocky” as it lost its exclusive grip on the iPhone and Verizon’s shares rose slightly on expectations that the iPhone would bump up profits, but the parent company’s stock price stayed fairly steady in the face of considerable declines in new customer growth.

If the trends we saw in AT&T and Verizon’s earnings continue – and it seems likely they will, given the market dynamics – the industry is going through another one of its major shifts. If anyone’s equipped to deal with lightning-fast changes in the marketplace, it’s operators. They’ll deal with this so-called “slowing” growth just fine.