Landline usage is in a decline. With the prevalence of wireless communications and VoIP, the number of landline subscribers is dwindling. The Center for Disease Control's Nation Health Interview Survey shows that 35.8 percent of American households have ditched the landline in favor of wireless choices.
More evidence is present that landlines are heading the way of the dinosaurs: just shy of 16 percent of American households said that they "received all or almost all calls on wireless telephones despite also having a landline telephone." When combined with the 36 percent above, you can extrapolate that more than half of the US doesn't use a landline.
With wireless coverage continuing to expand and improve, the trend will likely continue, with only mission-critical or privacy-concerned users continuing to use landlines.