More than a billion people use social platforms such as Facebook, eHarmony, Renren, and LinkedIn. What’s the attraction? They satisfy two basic human needs: to meet new people and to strengthen existing relationships. Fee-based dating websites, which collectively grossed $1 billion in 2010 by connecting strangers, now account for an estimated one in six new marriages. Facebook, which fortifies friendships, boasts a staggering 750 million users and a valuation in excess of $100 billion.

A version of this article appeared in the November 2011 issue of Harvard Business Review.