Why do people link?

. . . 1) reciprocator strategy – players propose links to those from whom they have received link proposals in the previous round; 2) myopic best response strategy – players aim to profit from maximisation; 3) opportunistic strategy – players reciprocate link proposals to those who have the largest number of connections. We find that these strategies explain approximately 76% of the observed choices. 

That is from the new paper "Behavioural patterns in social networks" by Conte, Di Cagno, and Sciubba.