San Jose, Calif., has the second-most single males per single female in the U.S.

More choice isn't necessarily a good thing when it comes to relationships, says Justin Garcia, a professor in the department of gender studies at Indiana University and a researcher at the Kinsey Institute for Research in Sex, Gender and Reproduction.

"Sometimes when you're in a big city and there are thousands and thousands of people you might find attractive, it becomes so overwhelming that you don't engage in the dating culture at all," he says.    

Overall, the Facebook data seems to back up Dr. Garcia's hypothesis: Big cities like San Francisco and New York had some of the lowest rates of coupling up. The data team also identified cities with high single male-to-female ratios and vice versa.

The most popular social network, it turns out, can also serve as the world's biggest survey. Facebook's 1.23 billion monthly active users world-wide constantly feed the company invaluable data.

An informal working group of stats whizzes and Ph.Ds on Facebook's data sciences team did the Valentine's Day analysis in October, using anonymous profile information gathered in the 50 U.S. cities with the most Facebook users. "This is a side project for us," says Mike Develin, the main researcher in the informal working group on relationships. "Forgive the pun, but it's our passion."

Facebook's data is by no means perfect. Some people don't bother updating the relationship status on their Facebook profile when they begin and end a relationship, for instance. And Facebook didn't break down the numbers by age or other demographics.

Overall, Facebook found that big, cosmopolitan cities with highly educated populations—places like New York City, Washington, D.C., and Miami—tend to have the highest percentages of single people. Those cities also tended to have the lowest couples rates.

A lot of cities with high relationship rates are in the South, Mr. Develin notes. Trailing Colorado Springs on the list are El Paso, Texas; Louisville, Ky., Fort Worth, Texas, and San Antonio.

That was no surprise to Philip Cohen, sociology professor the Maryland Population Research Center at the University of Maryland. The Facebook data says more about the age, ethnic makeup and education levels in those cities than whether they are singles meccas, he says.

He advises people in search of a relationship to go where there are lots of other single people, not where there are high couples rates. "You only need to form one relationship, but you might want a lot of singles to choose from," he said, in an email.

David Siegel, a 24-year-old violinist, doesn't fit the stereotype some people may associate with Colorado Springs, where he grew up. He moved back from New York over a year ago. "I've found it a good place to be single," Mr. Siegel says, citing the thriving bars, restaurants and art galleries.

New York's dating scene was a "rat race," he says. People in Colorado Springs seem more interested in a lasting relationship. "Playing the game has a lot less do it with it," he says.

Alejandro Ganem, a 28-year-old El Paso resident, said he often feels pressure to couple up—especially in a city big enough to support a vibrant night life but small enough for word to travel fast. "When I start dating a girl, immediately she starts hearing about me," he says, including things about his ex-girlfriends. Sometimes a woman will pressure him to change his Facebook status to signal that he's off the market, he says.

Not all cities with a high relationship ranking are in the South. Portland, Ore., is a city so progressive that it spawned a TV show lampooning its culture, and it has a higher relationship rank than Houston; Richmond, Va., and Nashville, Tenn. Portland also had a high single male-to-female ratio, beating even Anchorage, Alaska. Some of those cities weren't big enough to make the Top 50 ranking.

Single ladies, take note: Cities near outdoor recreation tend to have a high proportions of single men. Among places with the highest, Facebook says, are outdoor hubs like Livingston, Mont.; Boise, Idaho; and Vancouver, Wash.—which all were too small to make the Top 50 ranking—and, again, Colorado Springs.

Life in those towns isn't easy for single men. "It's horrible," says David Fischer, a 48-year-old Honeywell International Inc. project manager in Colorado Springs. He turned to eHarmony to find his future fiancée, Kara Galvin, who lives in Denver, about an hour's drive north. They plan to get married this summer.

Although researchers hypothesize that religious cities would rank higher in relationship rates, the Facebook analysis found exceptions. Charlotte, N.C., and Nashville are among the nation's most religious cities, according to the American Bible Association, but they both ranked low on Facebook's list of relationship cities.

El Paso and San Antonio may owe their spots in the top five relationship cities to their Mexican heritage, says Andrew Cherlin a sociology professor at Johns Hopkins University. "Hispanics tend to marry early," he said.

Eric Diaz, a 35-year-old dairy buyer at Whole Foods Market who grew up in San Antonio, says most of his friends met their spouses while in high school or college and got hitched in their 20s. He married Genefe Diaz, who moved to San Antonio after she graduated from college. When she arrived, many young professionals in town were already coupled up, she says. "San Antonio is just a really big small town," Ms. Diaz says. To find suitable men, "you really had to dig."