The obvious advantage of online dating is that it provides a wider pool of possibility and choice. In some respects, for the masses of grownups seeking mates, either for a night or for life, dating is an attempt to approximate the collegiate condition—that surfeit both of supply and demand, of information and authentication. A college campus is a habitat of abundance and access, with a fluid and fairly ruthless vetting apparatus. A city also has abundance and access, especially for the young, but as people pair off, and as they corral themselves, through profession, geography, and taste, into cliques and castes, the range of available mates shrinks. We run out of friends of friends and friends of friends of friends. You can get to thinking that the single ones are single for a reason.

If your herd is larger, your top choice is likely to be better, in theory, anyway. This can cause problems. When there is something better out there, you can’t help trying to find it. You fall prey to the tyranny of choice—the idea that people, when faced with too many options, find it harder to make a selection. If you are trying to choose a boyfriend out of a herd of thousands, you may choose none of them. Or you see someone until someone better comes along. The term for this is “trading up.” It can lead you to think that your opportunities are virtually infinite, and therefore to question what you have. It can turn people into products.

For some, of course, there is no end game; Internet dating can be sport, an end in itself. One guy told me he regarded it as “target practice”—a way to sharpen his skills. If you’re looking only to get laid, the industry’s algorithmic-matching pretense is of little account; you merely want to be cut loose in the corral. The Internet can arrange this for you.

But if you really are eager, to say nothing of desperate, for a long-term partner you may have to contend with something else—the tyranny of unwitting compromise. Often the people who go on the sites that promise you a match are so primed to find one that they jump at the first or the second or the third who comes along. The people who are looking may not be the people you are looking for. “It’s a selection problem when you round up a bunch of people who want to settle down,” Chris Coyne, one of the founders of a site called OK Cupid, told me. Some people are too picky, and others aren’t picky enough. Some hitters swing at every first pitch, and others always strike out looking. Many sites, either because of their methods or because of their reputations, tend to attract one or the other.

“Internet dating” is a bit of a misnomer. You don’t date online, you meet people online. It’s a search mechanism. The question is, is it a better one than, say, taking up hot yoga, attending a lot of book parties, or hitting happy hour at Tony Roma’s?

Match.com, one of the first Internet dating sites, went live in 1995. It is now the biggest dating site in the world and is itself the biggest aggregator of other dating sites; under the name Match, it owns thirty in all, and accounts for about a quarter of the revenues of its parent company, I.A.C., Barry Diller’s collection of media properties. In 2010, fee-based dating Web sites grossed over a billion dollars. According to a recent study commissioned by Match.com, online is now the third most common way for people to meet. (The most common are “through work/school” and “through friends/family.”) One in six new marriages is the result of meetings on Internet dating sites. (Nobody’s counting one-night stands.) For many people in their twenties, accustomed to conducting much of their social life online, it is no less natural a way to hook up than the church social or the night-club-bathroom line.

There are thousands of dating sites; the big ones, such as Match.com and eHarmony (among the fee-based services) and PlentyOfFish and OK Cupid (among the free ones), hog most of the traffic. Pay sites make money through monthly subscriptions; you can’t send or receive a message without one. Free sites rely on advertising. Mark Brooks, the editor of the trade magazine Online Personals Watch, said, “Starting a site is like starting a restaurant. It’s a sexy business, looks like fun, yet it’s hard to make money.” There is, as yet, a disconnect between success and profit. “The way these companies make money is not directly correlated to the utility that users get from the product,” Harj Taggar, a partner at the Silicon Valley seed fund Y Combinator, told me. “What they really should be doing is making money if they match you with people you like.”

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Some sites proceed from a simple gimmick. ScientificMatch attempts to pair people according to their DNA, and claims that this approach leads to a higher rate of female orgasms. A site called Ashley Madison notoriously connects cheating spouses. Howaboutwe.com asks only that you complete a sentence that begins “How about we . . .” with a suggestion for a first date, be it a Martini at the Carlyle or a canoe trip on the Gowanus Canal. (Your suggestion should theoretically be a sufficient signal of your taste and imagination, and an impetus for getting off-line as soon as possible. Apparently, a big winner has been a ride on the Staten Island Ferry.) The cutting edge is in mobile and location-based technology, such as Grindr, a smartphone app for gay men that tells subscribers when there are other willing subscribers in their vicinity. Many Internet dating companies, including Grindr, are trying to devise ways to make this kind of thing work for straight people, which means making it work for straight women, who may not need an app to know that they are surrounded by willing straight men.

Most of the Internet dating sites still rely, as TACT did, on the questionnaire. The raw material, in the matching process, is a mass of stated preference: your desire or intolerance for certain traits and characteristics. Many of the sites make do with that alone. The more sophisticated ones attempt to identify and exploit the dissonance between what you say you want and what you really appear to want, through the choices you make online.

“What you do is more important than what you say,” Greg Blatt, who is the C.E.O. of I.A.C., and a former C.E.O. of Match.com, told me. (Blatt not only runs the company; he’s also a client. He is one of those guys who say they enjoy dating.) You may specify that you’d like your date to be blond or tall or Jewish or a non-smoking Democrat, but you may have a habit of reaching out to pot-smoking South Asian Republicans. This is called “revealed preference,” and it is the essential element in Match’s algorithmic process. Match knows what’s right for you—even if it doesn’t really know you. After taking stock of your stated and revealed preferences, the software finds people on the site who have similar dissonances between the two, and uses their experiences to approximate what yours should be. You may have sent introductory messages to only two people, and marked a few others with a wink—a nonverbal expression of interest—but Match will have hundreds of people in its database who have done a lot more on the site, and whose behavior yours seems to resemble. From them, depending on the degree of correlation, the software extrapolates about you.