Today people turn to social networks for help with everything from finding a job to landing a date. So it’s only natural that virtual communities would crop up around another of life’s big challenges: losing weight.

But the usual suspects aren’t the ones leading the way in convening calorie counters online. That title belongs to SparkPeople.com, a free website that with 2.7 million unique monthly visitors, recently eclipsed Weightwatchers.com as the most popular dieting website.

SparkPeople isn’t new. Launched in 2001, the site claims to have helped users drop more than 10 million pounds, although most of its user growth has occurred more recently: Spark added 6,000 new members daily in 2009.

Its philosophy isn’t exactly novel either. Rather than plug a faddish, overnight diet plan (see Atkins, South Beach), the site pushes modest and flexible lifestyle changes via goal setting. It promotes fitness and healthy eating habits by offering tools like a calorie tracker, video demonstrations of exercises, and downloadable grocery lists.

While these assets are part of the site’s recipe for success, the key ingredient is its loyal and obsessive community. (It also helps that it’s free.)

Support groups are a mainstay of weight loss programs, but in the pre-Internet era they were limited to local gatherings of dieters, who sometimes had little in common other than geography. At SparkPeople members create profiles where they report on their progress and pen blog posts that help them stay accountable and inspire others.

One popular post is titled, “Old fat photos — 100 lbs heavier. If you need inspiration take a look!” Users can also leave notes of encouragement for their “SparkFriends,” peruse message boards, or join groups organized by members, most of whom are middle-aged women, around common interests or goals.

Weightwatchers.com offers similar features, but whereas the largest group on that site boasts 7,000 members, Sparkpeople’s group membership regularly numbers in the tens of thousands. Users also spend more than twice as much time on SparkPeople.com as their counterparts at Weightwatchers.com, according to Nielsen. This highly engaged community is SparkPeople’s primary source of growth: Members eagerly evangelize the site, trying to “spark” family and friends to sign up.

SparkPeople, which is not yet profitable, gets most of its high seven-figure revenue from advertising. It’s a lean operation, with 25 employees and only one outside investor: Steve Case, founder of AOL. Case says SparkPeople is unique because, “they’re creating more of a platform approach with tools, so users can customize their own set of goals to do things in a more permanent, sustaining way.” It also means the site is equipped to help users tackle goals beyond weight loss.

Jan Rea, 52, recently used the site’s goal-setting feature to complete her bachelor’s degree. Calling herself “addicted to SparkPeople,” Rea first joined the site five years ago to tackle weight loss and high blood pressure and cholesterol. Within two years she had lost 40 pounds and has kept it off since. Rea, who bought herself a fleece jacket with the SparkPeople logo, talks to anyone she can about the site and even posted a notice about it in her church bulletin.

“I always felt like I was starving with Weight Watchers,” she says of the company’s offline program. “With SparkPeople I don’t feel like I’m missing anything.”

Now that it’s the online leader in weight loss, SparkPeople is eying lucrative partnerships and offline licensing deals. Corporations, including IBM (IBM), have piloted versions of the site for employees, to help rein in skyrocketing health care costs.

In January, The Spark, a weight-loss book authored by company founder Chris Downie, who sold his previous startup to eBay (EBAY) in 1998, snagged the No. 1 spot on the Amazon.com, Walmart.com and B&N.com preorders lists, just prior to launch.

Now if only your SparkFriends could guard your refrigerator.