Lots of Civilisations?
Drake's formula summarises in an elegant way some of the most important factors you need to know in order to seriously discuss the existence and frequency of other civilisations. Drake himself did not expect this formula to become so famous when he was preparing an agenda for a meeting in 1961 at Green Bank.
There are several both objections as well as extensions to the formula. The concept of "habitable zone" have had to be extended since it seems probable that life sustainable environments may exist based on other energy sources than a star (cp the Jupiter moon Europa). Another modification is that a civilisation may possibly emerge several times on the same planet. Of course this depends on the life time of a civilisation but also on the durability of the planet in question. In case of Earth conditions for higher forms of life became favourable about a billion years ago but our planet should have similar conditions for another billion years.
The formula also does not take into account that civilisations may expand to neighbouring planetary systems. In addition, some think that the formula lacks a factor describing the will of a civilisation to really communicate actively, i.e. make specific efforts to contact other civilisations.
Of all the factors in the formula the one concerning the expected survival time of a civilisation is the one that mostly affects the result. In all cases, the enormous distances in the galaxy must always be considered and thus the simultaneity. It is not obvious that civilisations exist in the same epoch of time and thus have any chance at all of communication.