Through most of our history, we've assumed we occupy a privileged and unique place in the universe. In the 16th century, Copernicus paved the way for the understanding that we orbit but one star among many. Over the last two centuries, we've found that we're a small part of one galaxy, and that ours is but one galaxy among billions. If we look at those numbers, it seems ridiculous to assume ours is the only planet in that whole universe to host intelligent life.
Until the 1990's, the main counterargument was that planetary formation appeared to be limited to our sun. That changed in 1995, when radio astronomers were able to infer several extrasolar planets. The Kepler observatory was launched in 2009, and we've now verified the existence of nearly 2,000 exoplanets in more than 1,200 solar systems.
In 1960, Frank Drake decided to turn a radio telescope to the sky and listen for signals from possible alien civilizations. The experiment was underfunded and limited in scope. NASA took interest, and the search for extraterrestrial intelligence (SETI) was funded in 1971. It went through several iterations before funding was cut in 1992, with politicians calling it a "Martian chase" and a waste of money.
The Luddite claim was, "we haven't heard anything yet, so we must be alone." Light (and therefore, radio) travels at a finite velocity. We've only scanned a small part of a universe that's 13.7 billion years old and 46 billion light-years across. It's a bit like pronouncing evolution a sham after observing only one species for one generation.
For the last two decades, the SETI project has worked on a reduced scale and with fewer resources. It is currently funded by donations. Much of the data analysis is done through distributed computing via the SETI@home program.
Things took a turn for the hopeful this week, when Russian investor Yuri Milner announced he'd be donating $100 million to the project over the next decade. This gives SETI the resources to cover the sky in a breadth and depth they've never been able to manage.
Does this guarantee we'll hear something? No. Does it have monetary value? No. But we really need to stop thinking in those terms. We live in an age that's stopped valuing the pursuit of knowledge for its own sake. It's nice to know some people won't accept that mentality.