Nearly 280 million people, or 85.0% of the US population, will go online at least once a month in 2018, eMarketer estimates.

With smartphone adoption slowing, growth is expected to be less than 2% this year and remain low throughout the forecast period. The biggest shifts occurring among the US digital audience are no longer device-related, but rather, based on activities.

"At this point, just about everyone who wants a smartphone already has one," said Corey McNair, eMarketer forecasting analyst and author of the new report, "US Digital Users: eMarketer's Estimates for 2018." (eMarketer PRO subscribers can access the full report here.)

User numbers will rise just 4.2% in 2018 to 229.2 million people. Seniors ages 65 and older will be the only audience to experience a double-digit growth rate in smartphone use this year.

Approximately 83.0% of internet users will use a mobile phone to go online. Desktops/laptops are still widely used, as 84.0% of digital users are expected to use them to visit the web this year. Meanwhile,16.0% will exclusively use a mobile device for internet access.