The Pew Research Center has released U.S. Smartphone Use in 2015.
Here's an excerpt:
Today nearly two-thirds of Americans own a smartphone, and 19% of Americans rely to some degree on a smartphone for accessing online services and information and for staying connected to the world around them—either because they lack broadband at home, or because they have few options for online access other than their cell phone.
Indeed, 7% of Americans own a smartphone but have neither traditional broadband service at home, nor easily available alternatives for going online other than their cell phone.