Sarah C.P. Williams, writing for ScienceNOW:
To join Facebook or not to join Facebook? You might think the decision depends on how many of your friends are already on the social-networking site. But a new study reveals that it’s not the raw number of friends that matters but rather the types of friends who are signed up. The results are the first to show that groups of friends — rather than friend number — are important to how social trends spread.