The number of U.S. shares available to investors declined by 9.1 billion shares in 2016 after increasing in each of the prior four years. The majority of those shares, 7.5 billion, was removed from the market during the first half of the year as companies took advantage of suppressed equity valuations. The decrease in supply of equity shares, demand staying stable or increasing, has been partially attributed to the increase in equity valuations at the end of 2016 and into 2017.
The large drop from 2010 to 2011 was driven by a Citigroup 10-to-1 reverse share split in May of 2011 that took 24.9 billion shares out of the market. Without the reverse split, the market would have contracted by 6.4 billion shares.