The "length of time civilizations communicate" and stating that there are likely 140,000 civilizations right now, is really where the formula falls down IMO.
For one, we are not even sure we would recognize other forms of life. We know what Earth life is like, but we can't assume to know all the rules that would come into play in other environments. Never mind recognizing what their "civilization" or "communication" would look like.
Plus, it also assumes that the nature of time is the same everywhere or that civilzations have any longevity at all.
I think it was one of the first couple of episodes of the new Cosmos series where he points out that random chance played a big role on Earth. The above formula assumes patterns, which is really no different than looking in the sky and seeing constellations.
I am sure that there must be life out there, but I think it's a fools errand to try and quantify at this point.
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