Individual song downloads dropped from 1.34 billion in 2012 to 1.26 billion in 2013, the largest drop since the launch of the iTunes store in 2003, the music industry magazine Billboard reported.
Albums fared substantially better during 2013, with sales seeing a minor drop of 0.1% to 117.6 million units.
Industry experts have attributed the decrease to the rise of ad-supported and paid music subscription services - available for an average of $10 per month.
Experts said that the growth in streaming revenue was offsetting the decline in music downloads.
Billboard, which cited figures provided by the music sales tracker Nielsen Soundscan, also reported that CD sales dropped by 14.5% to 165.4 million units in 2013.
Vinyl continued its year-on-year ascent, rising from 4.6 million unit sales in 2012 to 6 million.