Another 21 percent met on social networks, while the rest got to know each other from a mixture of blogs, gaming sites, chat rooms, discussion groups and other online communities.
These couples tended to be happier in their relationships than couples who met offline, the researchers report this week in the journal Proceedings of the National Academy of Sciences.
"Our results indicate that of the continuing marriages, those in which respondents met their spouses online were rated as more satisfying than marriages that began in an offline meeting," said study researcher John Cacioppo, a psychologist at the University of Chicago.
A final possibility is that people open up more online than they do in face-to-face meetings.
Experimental lab studies have found that people are more willing to engage in "self-disclosure," or authentic discussions about themselves, when they meet online first.
"The observed differences in marital outcomes may not simply be the result of selection biases based on demographics," Cacioppo told Live Science.
The explanation for the differences remains a mystery.
This self-disclosure is linked to greater appeal and to firmer friendships in these studies.
Cacioppo and his colleagues also found that the location of face-to-face meetings correlated with couples’ happiness.
The most-satisfied married couples who met offline got to know each other through school, church, social gatherings or by growing up together.
The least-satisfied offline couples met through work, family, at bars or on blind dates.
Likewise, certain meeting spots on the Internet were more salutary than others.