Static formulations aside, there’s some good news about social and economic mobility. An article in the New York Times by social welfare professor Mark Rank lays it out:

The picture drawn of the 1 percent has been that of a static population, just as the 99 percent is often portrayed as unchanging. 

But, he writes:

It turns out that 12 percent of the population will find themselves in the top 1 percent of the income distribution for at least one year. What’s more, 39 percent of Americans will spend a year in the top 5 percent of the income distribution, 56 percent will find themselves in the top 10 percent, and a whopping 73 percent will spend a year in the top 20 percent of the income distribution.

As Rank notes, this suggests a higher degree of mobility – up and down – than is suggested by the class warriors.

Economist Mark Perry has additional analysis.