It almost goes without saying that the Fed's primary job is to ensure a stable currency. Most of the time, that means preventing inflation (unanticipated inflation, to be more precise), but sometimes it means preventing deflation.
Currently, the Fed's lever of choice for accomplishing that goal is the target interest rate banks charge each other to meet the Fed's reserve requirements. A target of 5.25% has been the policy for more than a year, but it might change tomorrow (9/18) when the Fed meets again.
When I sample internet forum discussions about money and the Fed, the "money supply" is mentioned frequently, because many people still equate a growing money supply to growing inflation. What is almost never mentioned, however, is "money demand"—which is curious, because when the supply of money grows in tandem with the demand for money to support the real economy, inflation is unaffected. Not only that, but the demand for money is what the Fed is strongly influencing when it targets the interest rate to implement monetary policy. Supply and demand together are just as important for monetary analysis as for goods and services.
A fact of life is that targeting the interest rate means giving up control of the money supply. Conversely, if the policy were to target the money supply, it would mean giving up control of the interest rate (...and high interest rates of the early '80s, when the Fed was targeting the money supply, are a good example). Here are two simple illustrations showing what the Fed can control (red line) and cannot control (yellow circle), as the demand for money grows steadily, as it must when the real economy grows steadily.
So today, with the Fed targeting the interest rate, the level of the money supply is an outcome, not a control variable. It's a result of the demand for money and the price of money (i.e., the interest rate). The Fed's job isn't easy in any case; measuring the inflation rate and growth rate accurately is difficult, and measuring the money supply is even more difficult if not impossible. Bottom line: the Fed's ability to gauge true inflation rates and real growth rates are much more to the point than the money supply level.
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For further reading, here's a concise summary, Formulating Monetary Policy, by William F. Hummel.