According to a report by the World Bank, three-quarters of the world population now has access to a mobile phone. At the rate this is going, it is predicted that by 2015, there will be more phones than there are human beings.

From the article in The Atlantic:

Even at the height of landline subscriptions there were "only" about one billion globally, and it took more than a century to get there. Of course, mobile and landlines require different infrastructure and most people did not have an individual landline but rather a household one. But even taking those differences into account, the spread of mobile is representative of a globalized economy in which new technologies can spread farther and faster than ever before.

Also, in contrast with landlines, mobile tech is not only a telephone but a broader multimedia device (just how broad depends on the phone). In particular, consider text messaging. Nearly five trillion (that's trillion, with a "tr") text messages were sent in 2010, more than 12 million per day. Mobile Internet use lags behind but is growing as more capable smartphones become cheaper and more widely available.