What happened to John D. Rockefeller's wealth?
American fortunes tend not to last longer than three generations; the Rockefellers' has remained substantial for 6 or 7 generations.
A big difference between American old money and European old money is, European fortunes tend to last a lot longer. "Primogeniture" is a law/custom whereby the oldest son (or, in the absence of sons, the oldest daughter) receives the entire family fortune and the other sons and daughters get a good education but not much else.
America doesn't have this custom. When a wealthy man dies, his children all get an equal cut (although there might be a provision in the will that cuts some of them out). If an American millionaire dies and his assets are equally divided between 4-6 heirs, each will have a substantial inheritance, but not a lordly one. And if each of these heirs in turn has 4-6 children (See: the Kennedys), the grandchildren will seriously have to work for a living.
The Rockefellers collectively control quite a lot of money (The precise amount is a closely guarded secret), with a labyrinth of trusts and philanhropic foundations. But John D. Rockefeller, Sr. has about 150 living heirs, few of whom wield titanic individual influence. Jay Rockefeller was a senator until last year and certainly made a lot less noise in that position than some.