Social targeting
Offered by various vendors, social targeting technology tracks a specific group of users (for example, your core audience) and then attempts to identify the online connections of these users (i.e., your friends and family). Once these connections are uncovered, media can be targeted to them via the same anonymous pixel technology that is currently being used to target online display ads.
Companies such as 33Across, Lotame, Media6Degrees, Rapleaf, and others each offer their own versions of this targeting. For example, Media6Degrees' product attempts to match the brands with their most engaged consumers based on its proprietary Social Signature methodology. Meanwhile, 33Across' SocialDNA system "helps marketers identify high-potential prospects who are socially connected to existing customers and brand loyalists."
I asked Eric Wheeler, CEO at 33Across and a well-known thought leader on this topic, about how his customers are thinking about paid social. "Earned and owned has been the primary social media focus for most brands thus far," he said. "Build a Facebook fan base, amass followers on Twitter, secure subscribers on YouTube -- all great ways to message to your existing and most loyal customer base.
"What is hard to accomplish with just this investment, however, is the ability to leverage those customer relationships and their social influence to reach your next customer," Wheeler added. "This is what social targeting enables. Imagine your Facebook fan base scaled by 30 times; your website visitors multiplying by the thousands. We find that once advertisers do dip into paid social, their investment in it scales along with their customer base."

From 33Across
Other companies, such as Clearspring and RadiumOne, have a twist on social targeting using technology to (anonymously) track what type of content users are sharing and who they're sharing too. Their bet is that people with whom you share are even more connected to you than someone on your Facebook friend list. From RadiumOne's website: "Do you know who your friends are? Are they really the 250 people you've listed as friends on a social network? Or is your circle of close personal connections actually much smaller? The fact is, a social network can be a bit like a giant address book, a way of keeping our contacts up-to-date. But the truly close -- and for marketers, valuable -- connections occur among people who are actively sharing their lives with each other."
As with all forms of targeting, there are effectiveness questions as well as privacy concerns with social targeting. However, if you're a marketer that's trying to reach the connections of your core customer base, this might be an avenue you should explore.