Harvard-U.C.L.A. researchers are investigating a concept, first put forth by the pioneering German sociologist Georg Simmel, known as triadic closure: whether one’s friends are also friends of one another. If this seems trivial, consider that a study in 2004 in The American Journal of Public Health suggested that adolescent girls who are socially isolated and whose friends are not friends with one another experienced more suicidal thoughts.

students who reported low satisfaction with life and low self-esteem, and who used Facebook intensively, accumulated a form of social capital linked to what sociologists call “weak ties.” A weak tie is a fellow classmate or someone you meet at a party, not a friend or family member. Weak ties are significant, scholars say, because they are likely to provide people with new perspectives and opportunities that they might not get from close friends and family.