Who buys stock? This answer may surprise you

Ulrich Baumgarten/Contributor | Ulrich Baumgarten | Getty Images

It's not surprising that the biggest buyers of U.S. stocks are U.S. companies through buybacks, but the sheer size of their purchases may be.

According to Citigroup, companies in the S&P 500, over the past 10 years, bought $4 trillion of their own stock.

GE became the latest Friday when it announced an eye-popping $50 billion buyback and asset sale, goosing its stock. That announcement is on par with the largest ever, Apple's $50 billion buyback announced two years ago.

Read MoreGE unveils massive restructuring, buyback