Are there other intelligent species out there?  Some people think that we are the most advanced civilization in the galaxy.

I think we're simply being nieve.  Think about it.  If you're an advanced civilization, do you want to advertise?  Do you want to send out radio signals that others can decode?

Or, perhaps, you want to be quiet.  Why?

Well, if there are more advanced civilizations out there and they are aggressive, do you want to invite them to come visit you?  If the Native American's knew what the Europeans were going to do to them, would the Indians have helped the Mayflower settlers survive?

Now, think about cryptography.  The best crypto algorithms make meaningful signals appear like white noise.  And we're just 35 years into the likes of RSA cryptography.  Imagine the kind of cryptography we'll have in 50 or 100 or 500 years.  All our communications will appear as white noise to any listener.

Perhaps every civilization goes through the "hey, we're here... come visit us" phase when they have sub-light speed communications technology.  Maybe some of them get welcome visits.  Perhaps others get unwelcome visits.  But I'm betting by the time civilizations get supra-light speed communications and transportation technology, they stop advertising.  They use exclusively encrypted communications technology.  Those folks are not going to be visible to us.  Yeah, maybe we'll get some civilization's version of I love Lucy in a radio spectrum.  But I'll bet by 2050, every form of communications on earth will appear as white noise to an outside observer.

So, in order to get a more realistic probability that we'd observe another intelligent civilization with SETI, we should be estimating that radio signals will only be coming from a civilization for about 150 years and then all the radio signals will appear as white noise.  So, even if there are 10M civilizations in our galaxy and those civilizations have existed over 1B years, but they only broadcast non-white-noise signals for 150 years, what's the probability of us hearing one of their signals.  That number is pretty low.

I believe there are a fair number of advanced civilizations in our galaxy, but I don't think they are hanging out a "welcome" sign.