The quartet spun in a tight pace line through much of the ride, working as an energy-efficient team as they covered 30 miles of rolling hills, steep climbs and hair-raising descents.

But the riders hardly knew one another. They organized the ride through BayAreaLinkup.com, a Walnut Creek-based Web site that for a $4.95 monthly fee links people with common interests, whether biking, hiking or singing along to Beatles songs.

"It's about the best $5 I spend each month," said attorney Barry Finkelstein, 44, an Alameda resident who arrived on a blue Trek 30-speed he bought the day before. "You can't get four friends together for a bike ride, but you can get six strangers."

Linkup

Bay Area Linkup -- dubbed BAL by its members -- falls into the broad category of social networking Web sites, a red-hot sector that allows users to connect online and create much of the content themselves. Companies like MySpace, Facebook.com and Classmate have drawn huge audiences as well as the eyes of venture capitalists, Wall Street and media giants.

"I think it's being propelled by the need to provide some kind of counterbalancing force to the alienating aspect of the Internet," said Michael Tchong, trend analyst with San Francisco-based Trendsetters.com. "The urge to bond socially is important across age groups."

BAL occupies a niche within social networking, alongside UrbanDiversion.com, Meetup.com or online dating sites, aimed at turning virtual relationships into real ones. It is not a singles group per se, but many members are single and coupling up does occur.

Social events sponsored by members range from the common -- dinner, movies, concerts -- to the highly creative. Event titles for just last week included: The Truth About Bats, Tango in the Library, "Whose Line is it Anyway?" Marathon and Discuss the Ballot over Coffee.

With 20,000 users, compared with MySpace's 55.8 million, BAL isn't exactly positioned for the next multimillion-dollar takeover. But the two-person operation is profitable and it is growing. There are nascent Linkup sites in 29 other cities, from New York to Hong Kong, with an additional pair set to switch on in Australia and New Zealand during the next few weeks.

Founder Fírinn Taisdeal, who started the service more than three years ago as a small lunch group, said he doesn't want BAL to be the next MySpace. He insists his priority is ensuring it remains a friendly, safe, useful community. Toward that end, he conducts short interviews with each prospective member, monitors event reports from hosts and kicks out anyone who behaves inappropriately -- all measures that effectively limit the service's growth.

"Given the choice between 10,000 people you can't rely on and 10 you can," Taisdeal said, "the choice is clear."

'Flake Factor'

Reliability is a theme he returns to again and again. Taisdeal quickly found that "flakes" -- people who sign up for an event but don't show up -- stifle the enthusiasm of hosts and guests alike. It creates a sort of negative feedback loop, where fewer and fewer people show up, fewer and fewer people host and so on.

That's why he devised the so-called "Flake Factor," which rates members according to their attendance records.

The lower the score, the fewer events he or she can attend. If the user reaches zero, after five closely timed flakes, he or she is booted from Linkup.

The system effectively discourages most members from leaving hosts in the lurch.

"I love it," said Janeen Olsen of Vallejo, a wine marketing professor who organized the bicycle ride and regularly puts together dinners. "As a host, and especially for the type of events I have, I would feel really bad if no one showed up."

But not everyone likes the Flake Factor, which can be applied unevenly, based as much on the personality of the host as the infraction itself. At least one former member complains that Taisdeal kicked her out unfairly and without opportunity for appeal.

Another problem with the Linkup system, some say, is that there's no foolproof way to screen users. Earlier this month, for instance, a Boston member complained that a man at an event, at least 20 years her senior, kept touching her inappropriately.

"There are 6,000 people in the system just in the Bay Area and many more outside," Taisdeal said. "It's like a small town, so you have people do things that are not acceptable."

The best he can do is kick them out after the fact.

Connecting

On the Saturday night of Oktoberfest weekend, as streams of twentysomethings sought out or stumbled from the beer hall at Fort Mason in San Francisco, a few dozen BAL members assembled in a small second-floor room in Building B.

Ages ranged from mid-30s to early 60s and looks spanned San Francisco hipster to librarian smart. Initially, a crescent of filled seats faced the Linkup band, playing a hodgepodge of guitars, drums, basses and tambourines that spoke more of availability than acoustics.

By the fourth song, as members started on their second or third plastic cup of wine, the gap between audience and performer began to blur.

After the a cappella lead in to the Beatles' "All My Loving," half a dozen people stood to dance as they belted out the lyrics projected onto the wall. By the downbeat of "Roll Over Beethoven," the whole room was twisting and shouting. More shakers and tambourines appeared. Someone passed out kazoos.

It's tempting to dismiss the gathering as cheesy, a little sad or at least not as cool as those kids staggering through Fort Mason's parking lot. But members wore unembarrassed smiles as they swayed or bobbed or proudly sang out of tune.

Discussing why they joined the group, several articulated an infrequently spoken truth: At a certain point in life, social circles shrink and making new friends becomes increasingly hard for many. College pals fan out across the country, married friends fade into domestic life, romantic relationships end, work friends remain at arm's length.

Linkup engineers occasions to exercise the social instinct and, on occasion, forge new and lasting bonds.

"People truly yearn to be connected," said Paul Janoff, 49, of Hercules, who plucked out the electric guitar solos during the evening. "The people you meet at the gym or church, those relationships are very superficial. But if you sing together or hike five miles, there are bridges built that are hard to break."

Reach James Temple at 925-977-8534 or jtemple@cctimes.com.