He owns about 14% of it. He has a majority of the voting stock, and presumably so will his heir. But most of the company is owned by someone else.

I was about to respond that there is no way the class B stock could be passed down to an heir with 10x voting rights attached, but after reading the documentation it does. So his family could run it into the ground at their leisure! Though Mark would have the ability to convert it from class B to class A when passing the stock down if he wanted.

This also highlights an extreme imbalance between ability to do great harm and bearing the brunt of said harm.