Mark Granovetter is famous for his ideas on weak ties within social networks. Although his most famous work is over 40 years old, it’s still relevant nowadays. I’ll start with an outline of his main ideas, if you want more detail I recommend you read the original article in the American Journal of Sociology (1973).
Granovetter starts with the premise that within social networks there are different degrees of relationships. He suggests that intuitively we recognize our relationships as strong, weak or absent. Two people who share a strong tie are likely to spend time together and share interests as well as character traits. They are also likely to share friends and relationships. This leads to the development of a close network of people.