Taking all these factors into account means that a lower estimate can be made. There's likely to be more than the 50 billion exoplanets Borucki describes.
Making this estimate is a relatively simple task, not so simple is estimating how many of these worlds might play host to life. As we know that only one planet in the Milky Way has life on it (Earth, in case you were wondering), no amount of statistical guesswork can arrive at an estimation for the number of alien beings that are out there.
Making estimates may sound trivial, but it does put the search for ET into perspective. There's at least 50 billion worlds, which have fostered the development of basic lifeforms? How many have allowed advanced civilizations to evolve?
If there are any space-faring alien races out there, "the next question is why haven't they visited us?" Borucki asked. He responded with: "I don't know."
I wonder if we'll ever know.
Image: An infrared observation of the core of the Milky Way as imaged by the Spitzer Space Telescope (NASA/JPL-Caltech/S. Stolovy SSC/Caltech)