We generally talk about the rise of broadband Internet in two stages. First there was dial-up, then there was broadband. Boom. That's it. Along the way, the proportion of Americans with one technology or the other has fluctuated more or less predictably. As more people signed on to broadband, fewer were left on dial-up, leading to talk of a "digital divide" between a wealthy, urban class and the elderly, rural poor.

But that narrative tends to overlook a really big part of the American Internet market. It turns out that there's another group of underserved Internet users in the United States: Those who have broadband Internet, but only through their smartphones — not through traditional wireline services like cable or fiber or DSL.

The share of Americans who access broadband only from mobile devices has reached 10 percent, according to new data from the Pew Research Center. Most of the people in this group are young, have never gone to college and make less than $30,000 a year. It's easy to think that smartphones are an accessory, an addition to your home WiFi for when you're out and about. For tens of millions of Americans, however, a smartphone is their only Internet lifeline, precisely because a phone is so much more versatile than a fixed Internet connection.

As anyone who's used a smartphone knows, mobile-optimized Web sites aren't exactly the most functional. For detail-oriented tasks like filling out forms and editing documents, a full-size computer is still your ideal machine. And as more government and commercial services move online, not having access to wireline broadband could become problematic.

That said, the rise of mobile-only Internet users might become another bit of ammunition in a wider debate over whether the PC era is nearing its end. With many smartphones, it's also possible to transmit your 3G or 4G data connection as a WiFi hotspot. That means that as data services become more reliable, tethering your phone to a laptop might become a real alternative to buying a traditional subscription from an Internet service provider.

Other regions have shown that they're perfectly fine getting by on mobile broadband. Just 7 percent of households in Africa have fixed broadband, according to the International Telecommunication Union. Yet mobile broadband penetration is at 11 percent, reflecting a compound annual growth rate of 82 percent over the last three years. In other words, Africa seems to be leapfrogging the wired infrastructure that dominates the West and moving straight to the next technology.