Quantitative Easing (both I and II) has caused the monetary base—the sum of currency and bank reserves—to explode in the past three years, but has not resulted in similarly large increases in the growth of broader measures of the money supply such as M2. Instead banks have largely held the extra money that the Fed created in order to finance its purchases of longer term Treasuries and mortgage backed securities. You can see this in the following time-series chart. As the monetary base (right scale) increased sharply, the ratio of M2 to the monetary base—the M2 multiplier (left scale)—has moved in the opposite direction in complete lock-step fashion. Thus changes in the multiplier have offset increases in the monetary base.
But if you look closely at the lower right of the graph, you can see that this pattern may have shifted recently as the M2 multiplier increased.