Dunbar’s number is the supposed cognitive limit to the number of individuals with whom any one person can maintain stable social relationships.
In a 1992 article, Dunbar used the correlation observed for non-human primates* to predict a social group size for humans and using a “simple” regression equation on data for 38 primate genera, Dunbar predicted a human “mean group size” of 150 (with 95% confidence interval of 100 to 230).
Dunbar’s Friends is my definition (and trademark ;-) to those few “real”, trusted and known people in your huge** online social network***.
* Primatologists have noted that, due to their highly social nature, non-human primates have to maintain personal contact with the other members of their social group, usually through grooming (and not “poking” as you might be expecting :-). The number of social group members a primate can track appears to be limited by the volume of the neocortex region of their brain.
*** Social Network for Dummies – Lee and Sachi LeFever (a.k.a. the CommonCraft‘s family :) have created a wonderful video explaining social network in plain English.