I would argue that there is simply no way for us to know this. It may be possible for government agencies, with their access to intelligence reports, to know whether terrorist organizations are getting weaker, but those of us without security clearances have no way of knowing whether the US is really being effective in reducing the danger of terrorism.
First of all, how would we know if we were reducing the danger of terrorism? We know that there were some attempted attacks (shoe bomber, underwear bomber) on airplanes coming into the US in past years but there have been none (that we know of) since. However, we have recently had the Boston Marathon bombing. Does that mean terrorism is being reduced or not? There is no good metric for measuring this.
Second, we in the public do not know how many real threats there have been and how strong terrorist groups are. For example, there was recently a strong alert about the possibility of attacks worldwide. However, nothing happened. Was that because something was done to prevent the attacks or was there never really any threat?
We can clearly see that the US is trying to do a variety of things that the government thinks will reduce or prevent terrorism. However, there is no way to know if these efforts have been successful.