Estimating the number of intelligent species based on the lack of obvious evidence is always dangerous. It's like primitive jungle tribes in 2009 assuming that advanced communication would consist of using very large drums.

Another issue: Assume we (earthlings) are the star-faring intelligence in question. Interstellar probes, barring some fantasy "warp drive," would require hundreds of years to deal with just the local stellar area. Although in SF you run across an Earth-like world every time you turn around, odds of one being within this local area are pretty slim. So barring Earth II, what reason would you have for colonizing those systems? If you had the ability to terraform, there are some worlds in our own system we can use. If you colonize open space using free colonies and asteroids, you have the entire volume between Venus and the Belt. To start with.

In other words, there may not be a lot of reasons to START an interstellar "colonization wave" in the first place. And even if you did, it would probably be free-space colonists (not planetdwellers) who carried it out, which means they'd never really bother with the planets in the other systems anyway.