True love is hard to find. As a result, people have traditionally sought help from matchmakers, friends and family. Arranged marriages and blind dates, church socials and dinner parties all rely on real-world social networks. We depend on the connections of others.

But these methods are quickly becoming obsolete. Nearly 20% of all relationships now start online, while a recent survey of baby boomers found that almost a third of new couples met on a dating website. EHarmony.com...