"The problem, of course, is that none of the terms can be known, and most cannot even be estimated. The only way to work the equation is to fill in with guesses. [...] As a result, the Drake equation can have any value from "billions and billions" to zero. An expression that can mean anything means nothing. Speaking precisely, the Drake equation is literally meaningless..."
M Crichton
And he writes a lot of fiction.
Honestly, it's very interesting stuff to read about, but as Mal said, there's way too many unknowns involved for it to stand up to proper scrutiny.