In a terrific 2003 New York Times article by Amy Harmon, a fourth-grade teacher, retold the statistics of her four-months of online dating: messages exchanged with 120 men, phone calls with 20, in-person meetings with 11—and 0 relationships. That's not efficient at producing relationships—but it is efficient at producing anxiety. My favorite sentence from that article:
It's amazing how all women say they're slender when a lot of them are overweight," said one 79-year-old Manhattan man who lists himself as 69 on his Match.com profile.
On the other hand, back in the days of dating, women entering college in the 1950s reported an average of about 12 dates per month (three per week) with five different men. These women were grossly outnumbered in college, and most women didn't go to college, so it wasn't a system for the whole society. But it tells us something about efficiency: Since dating reliably ended in marriage within a few years, it was pretty efficient, but that's because of the attitude and expectations, not the technology.
For people who are intent on being choosy, online dating might be more efficient than meeting people in person, but people in urban areas have been finding alternative partners for a long time. For example, we have known for several decades that people are more likely to divorce when they are presented with more, or better, alternatives. In the 1990s researchers discovered that "the risk of [marital] dissolution is highest where either wives or husbands encounter an abundance of spousal alternatives." They concluded, "many persons remain open to alternative relationships even while married." This has been shown not only by looking at the composition of the surrounding urban area, but also by simply comparing the divorce rates of people who work in gender-mixed versus gender-segregated occupations (the former are more likely to divorce). Marriage hasn't been unleavable for quite a while.
Still, maybe online dating speeds up the turnover process, and this might contribute to the trend of delaying marriage going on since the 1950s.
Second, I think it's possible that—in addition to undermining what's left of monogamy—the spread of online dating will widen some social inequalities. Remember those left behind by Jacob's wandering webcam eye in the article? When he wanders off to a new partner, he leaves one behind. She might or might not have the same options to exercise. In this rapid-turnover process, the richer, better-looking, healthier, better-lying, etc., might make things miserable for more people than they used to be able to. Jacob's efficiency might be their wasted months and years.
But remember, divorce rates have probably been falling more or less continuously since about 1980. And it is the less well-off who have been marrying less and divorcing (relatively) more. The people who are divorcing more—or marrying less—are the ones who aren't going to do as well in the "efficient" competition on dating sites. They aren't going to gain much from this onlinification.