|A generic depiction of a social network.|
In 1967, American social psychologist Stanley Milgram completed his “letter mailing” experiment, otherwise known as the small-world experiment, as part of his dissertation work at Harvard, wherein he found that people are connected socially on average by about six links distally from one another, hence the phrase ‘six degrees of separation’. 
In the late 1960s, American sociologist Mark Granovetter obtained data for his doctoral thesis at Harvard University by interviewing dozens of people to find out how social networks are used to land new jobs. Granovetter found that most jobs were found through "weak" acquaintances. This pattern reminded Granovetter of his freshman chemistry lesson that demonstrated how "weak" hydrogen bonds hold huge water molecules together, which are themselves held together by "strong" covalent bonds. A similar combination of strong and weak bonds, according to Granovetter, holds the members of society together. This work culminated in the publishing of his famous and highly-cited 1973 article "The Strength of the Weak Ties".
In 2010, Medical sociologist Nicholas Christakis began to explain and model social networks using carbon atom bonding models, using the motto “like atoms in a molecule, we’re all linked together.”
1. (a) Gladwell, Malcom. (2000). The Tipping Point (pgs. 34-36). Little, Brown, and Co.
(b) Stanley Milgram – Wikipedia.
● Social network – Wikipedia.