How many people in the US actively invest in real estate?

[Hunter B.]

The more I learn about real estate, the more I realize how little most people know about it. It makes me wonder: How many people are there that do this in a serious way? I'm not talking about REITs. How many people actively own cash flowing properties or make a significant profit on real estate every year in the US?

My guess: Random reference group of 30 people I know, 2 are real estate investors. That means ~6.67% of people invest in real estate. I would guess 20 million people in the US actively invest in real estate. 

When I put it that way, it actually makes me think there are a lot of us.