Do postdocs in the United States generally get annual raises?

From my experience in the US, it is typical for a postdoc to be on a fixed "short term" contract where there are certainly no "promotion" raises, but might or might not provide cost-of-living increases.

A one-year contract will almost certainly contain no salary increase; for a 2- or 3-year contract, they will be minimal at best.

In my experience (U.S., both East and West coasts) where postdocs are state employees, they will get whatever nominal raise is given across the board to state employees, if such a raise occurs. This is what happened both when I was a postdoc, and it is standard practice for us to build small raises into the budget for postdocs.