Nielsen Music started tracking digital song sales in 2003, the same year that Apple's iTunes Store launched.
("Plan" peaked with 127,000 downloads sold in its debut week, on charts dated Feb. 3.)
As recently as two years ago, Adele set the record for the most downloads sold in a week: 1.11 million for "Hello" (Nov. 14, 2015). The smash stands as an anomaly, however, as, previously, Flo Rida's "Right Round" had held the mark of 636,000, logged on the chart dated Feb. 28, 2009. (Concurrent with the record-setting frame for "Hello," parent album 25 launched at No. 1 on the Billboard 200 with a Nielsen-era record 3.38 million copies sold in the U.S.)
The latest sum for "Plan" further reflects the decline of download sales, as streaming continues to surge. Notably, "Plan" tops the Streaming Songs chart with 59.6 million U.S. streams in the week ending March 15, and all of the song's first eight weeks rank among the top 11 streaming weeks for any song all-time.
So far in 2018 (through the week ending March 15), digital song sales are down 27 percent compared to the same year-to-date point a year ago (99.3 million vs. 135.2 million, as of the week ending March 16, 2017).
Overall digital song sales totaled 554.82 million in 2017, down 23 percent compared to 2016 (724.04 million). 2017 marked the fifth straight year that digital song sales declined. That's an unsurprising distinction, considering how many consumers transitioned to enjoying music through streaming services instead of via purchasing albums and songs; overall on-demand streams (audio and video combined) climbed 43 percent in 2017 to 618.03 billion.
From 2004 through 2012, digital song sales grew on a yearly basis. The high-water mark for the format's sales was 2012, when song download sales totaled 1.336 billion.