The famous axiom 'the enemy of my enemy is my friend' is part of Austrian psychologist Fritz Heider's social balance theory, introduced in the 1940s. Previous studies have tried to model social networks based in famous theory but results remained controversial. New model takes into account two key pieces simultaneously: Not everyone knows everyone else in a social network, and some people are friendlier than others. With those two constraints, large-scale social networks consistently align with social balance theory. Model has broad applications for exploring political polarization, neural networks, drug interactions and more.