One of my favorite authors, Stephen Baker (he wrote Numerati), just penned the cover story for the June 1 edition of BusinessWeek:    Learning, and Profiting, from Online Friendships

This is a really interesting article on the value of friends and how marketers and others can effective use this information to better serve their consumers.   I like how Baker tackles this issue.   And here he mentions Rapleaf:

In an industry where the majority of ads go unclicked, even a small
boost can make a big difference. One San Francisco advertising company,
Rapleaf, carried out a friend-based campaign for a credit-card company
that wanted to sell bank products to existing customers. Tailoring
offers based on friends' responses helped lift the average click rate
from 0.9% to 2.7%. Although 97.3% of the people surfed past the ads,
the click rate still tripled.

Rapleaf, which has harvested data from blogs, online forums, and
social networks, says it follows the network behavior of 480 million
people. It furnishes friendship data to help customers fine-tune their
promotions. Its studies indicate borrowers are a better bet if their
friends have higher credit ratings. This might mean a home buyer with a
middling credit risk score of 550 should be treated as closer to 600 if
most of his or her friends are in that range, says Rapleaf CEO Auren
Hoffman.