Sagan forgot to put a time constraint on the calculation that made to predict intelligent life. To really properly calculate the probability, you would have to take into account the amount of time it took intelligent life to occur on our planet and set it in conjunction to the relative age of the other habitable worlds. As a very poor calculation, one could say that it is probable that only half of the calculated planets would have gained intelligent life before us.
Also, with how large our galaxy is, the time it takes for a signal to reach us must also be taken into consideration. An average civilization would have to be transmitting a signal thousands of years before we would receive anything. This would reduce the amount of planets yet again, albeit not significantly due to the vastness of geologic time.
Not that it really matters anyway, since it is basically impossible to calculate anything like this accurately while using tremendous assumptions.