Mimicking Facebook, which opened its service to outside developers several months ago, Google’s OpenSocial system gives developers standardized tools to write applications and embed them on social networkers’ personal pages.

By way of example, last week I started a Facebook group for executives at independent online agencies. This is a very narrow audience in every sense of the phrase. I had two dozen friends from my list of connections that I invited to join the group. Those who accepted the invitation kicked off a viral effect that would be tough to turn off even if I wanted to. Their friends saw that they joined the group. Since independent agency executives know other independent agency executives, within a day I started getting requests from people I didn’t even know to join the group.

Now, rather than doing some sort of expensive outbound communication (like a targeted email), I instead have qualified people coming to me. Apply these marketing principles to a for-profit endeavor and you’ve got a formula for launching targeted marketing programs that don’t cost very much.