LONG BEFORE THE founders of YouTube sold out for $1.65 billion and the creators of MySpace cashed in for $580 million, there was Andrew Weinreich. A pioneer in the social-networking business, Weinreich founded SixDegrees.com more than a decade ago. The online community for young adults began as an idea scribbled on paper. Four years later, in 2000, he sold it for $125 million.

"The idea was that I could change the way people interacted," says Weinreich, whose site allowed users to connect through each other's contact lists — an approach that's currently driving legions of fans to Facebook, LinkedIn and other sites. "That was a fascinating concept to me."

Today, Weinreich is back in the social-networking game with MeetMoi.com, a location-based mobile dating service. Only now, he's far from the only game in town. A growing number of entrepreneurs are launching social-networking sites, capitalizing on a public appetite for online connections and investors' desire to fund such ventures.

"We have seen a relatively significant increase in these sorts of sites," says Glenn Okun, a professor of management and entrepreneurship at New York University's Stern School of Business. "It's a major trend that has been underway."

In fact, the winners of NYU's annual business plan competition this year — CollegeHotList.com and Comixology.com — are social-networking sites.

"Social networking is really a buzzword right now," says David Steinberger of Comixology, a social-networking site for comic-book lovers. "Our goal is to have better information about comic books and tap into this passionate group of people that love to talk, write and think about comics."

Like other social-networking sites, Comixology allows members to create profiles, post blogs and weigh in with chatty commentary (in this case, about their favorite comic books). It's easier — and cheaper — for such sites to let users develop the content, rather than staff. It's also an increasingly effective way to draw traffic.

"You're playing into the voyeuristic tendency that we all have, to watch and comment on others from behind the computer screen," says Lori Hoberman, venture-capital expert and co-chair of the corporate and securities group in Fish & Richardson's New York office. "It creates a channel for advertisers and, therefore, a revenue stream for a potential company. That's where the investors see the value."

However, experts stress that entrepreneurs who want to build social-networking sites should seek more than just advertising to sustain a lucrative business.

"When I look at Internet-based business plans, I'm always looking for truly distinct multiple revenue sources," Okun says. "The pure advertising-based approach is a low-probability route of success in and of itself."

Take CollegeHotList, which serves as a college social-life organizer, helping students connect with each other, as well as to parties and venues. Aside from advertising, the site plans to get revenue from text-messaging features, commissions on product purchases, and market research.

The site's features include "social mapping," where users can view a college region, search by class (such as freshman, seniors or alumni) and check out where the guys or girls are headed that evening.

Tips for Social-Networking Start-Ups

1. Know your demographic. Find a way to fill the needs of a market that's not being served. "The more specific of a niche that you have for your social-networking site, the more likely that people will find you no matter what," says Nancy Slotnick of Cablight.com.

2. Develop a money-making strategy. Keep in mind, most web sites don't generate enough traffic to be economically viable based on advertising alone. "Provide some additional value that you can charge for, either on a usage or subscription basis," says Glenn Okun, professor at NYU's Stern.

3. Know your limitations. Be willing to give up control of certain aspects of the business to grow your network. "You need to figure out how you can build teams, how you can work with people, and when you're going to be able to do that," says Ben Taylor of Enpresence.com.

4. Remember, it's a balancing act. There's always a risk of criticism when you allow users to comment and contribute to your site. "It's interesting to see if other people believe in your product, and if they don't then maybe there are useful suggestions to make it better," says venture-capital expert Lori Hoberman.

5. Stay focused, yet willing to adapt. It takes a lot of commitment and motivation to do whatever it takes to implement your creative ideas. "But you don't want to overlook things, like taking into account how things are changing," Taylor says. "You want to have a strategy, and stay true to that strategy, but be flexible."

Yet even such a unique feature won't prove useful, if there aren't enough members to search for, acknowledges Chris Mirabile, co-founder of CollegeHotList. That's why the site also provides content to attract members and keep them.

"We have over 20,000 venues uploaded to our site, including videos so you can see the crowd at a venue, hear the music and listen to interviews of the patrons and bartenders," Mirabile says. "It's content that college students can benefit from, so this will keep them using the site until we accumulate the critical mass of users that's necessary."

Like other sites in the sector, CollegeHotList has caught the eye of venture capitalists, says Mirabile, who's looking into a first round of outside funding. VC firms are drawn to high-growth businesses and recent statistics from research firm IDC predict that the social-networking market will grow to $428.3 million in 2009 from $46.8 million in 2006.

Nancy Slotnick hopes venture capital will propel her love-life management site, Cablight.com, to new heights. An author and dating coach, Slotnick is relaunching the site in November as a social network for women to share dating experiences and learn how and where to meet men. It will include an online TV show, called Cablight Girls, featuring real-life trials and tribulations of the dating scene.

"I feel the timing is right with social networks that I want to strike while the iron is hot," says Slotnick, who was about to close on her first round of venture capital at press time. (She declined to provide the dollar amount.) "The competition is tough and if you don't have capital behind you, the big players can swallow you."

Others are not so willing to take VC, betting they can achieve success without having to give up equity.

"Typically, a venture fund is going to seek a third of the company. If you're looking at $5 million, and the company is only worth a million then they just bought you out completely," says Ben Taylor, founder of Enpresence.com, a mobile social-networking site that connects people through their cellphones. Although Taylor has used venture capital in previous business ventures, he's not eager this time around.

"In the beginning it can be good to have control because you can see things differently than the investor," he says. "But later on, if it's successful beyond a certain point, then you need to think of it in terms of an investment and understand what you're getting out of it."

Regardless of whether they take on investors, a big concern for social-networking sites is keeping costs down during the early stages when they must attract members. Thanks to the popularity of the industry, more vendors are offering services that entrepreneurs can try. For instance, Ning.com and KickApps.com design social-network platforms for free or a nominal fee.

"You may have to share or give up the advertising revenue, but you can build your membership base, then get funding based on a web site that's viable and is getting advertising," says Slotnick, who's considering using a vendor for her site.

Weinrich, the SixDegrees.com founder, says much has changed since the late '90s when he quit his job and maxed out credit cards to support his first social-networking venture. For MeetMoi, he has secured $1.5 million in venture capital this year and plans to incorporate new technologies as he builds the business.

"If you have a niche, a different marketing idea or business model and you're excited by social networking, it's as good a time to start a social network now as it was two years ago, as it will be in five years," says Weinreich, who still determinedly carries a notebook everywhere he goes to capture oft-inspired ideas. "I don't think this game gets played out."