Technology Review reports: "A couple of years ago, Sandy Pentland, professor of media arts and sciences at MIT, handed out about 100 Nokia cell phones to MIT students and faculty. The phones were equipped with software that helped Pentland's team log interactions between the people carrying them. Based on phone calls and the devices' physical proximity to other people's phones (as measured by Bluetooth), Pentland and researcher Nathan Eagle developed social-network models that were more accurate and more nuanced than those constructed from the subjects' self-reports. A paper on the study is currently under review at the journal Nature."
IMHO: You have to sacrifice to privacy for this type of service in my opinion. If you read the article you will understand my last comment.