A growing number of social networks are racing to capture the loyalties of the Web's youngest and most vulnerable users.
Last month, Silicon Valley-based Kazaana launched with $3 million in funding and with hopes of attracting subscribers younger than 13 who will be addicted to friending, photo-sharing and farm-tending games. Kidzworld, with its 2.2 million users, has signed up advertisers such as Crocs and Crate & Barrel. Yoursphere boasts AT&T and the American Youth Soccer Organization as paying sponsors who want to lend their services to the site's 500,000 subscribers -- from preschoolers to 17-year-olds.
Then, of course, there's Facebook, which is pondering whether to allow preteens to join its massive social network with their parents' permission.
On the fickle Web, most of the smaller social networks may not last, experts say. But Facebook and its rivals hope some of their staying power will come from children who will commit now and remain with them through the years.
"Facebook probably isn't going to make a lot of money advertising to children, but the strategy is to get children to use the platform before they get pulled away by others," said Simon Mansell, chief executive of TBG Digital, an advertising and consulting firm for big corporate brands on Facebook and Twitter.
The rush to capture young users has sparked fresh concerns from lawmakers and health advocates over whether social networks will keep children in front of computer and mobile-gadget screens for too long.
They also worry about online predators.
In June, Skout, an app designed for flirtation with local strangers, said adults accused of sexually assaulting youths had lied about their ages to gain access to the site's teen section. Before those incidents, the company did not verify users' ages. Facebook has also had to step up its monitoring of sexual predators. Facebook did not respond to requests for comments for this article.
Networks that cater to children tout themselves as safe environments, although they acknowledge that there may be no foolproof way to block predators. Yoursphere, for instance, says it does a thorough background check on adults who register their children for the site.
Under law, the sites must seek permission from parents to sign up users younger than 13. Facebook restricts users under that age, but an estimated 6 million underage users have lied about their age to get on the site, consumer groups say.
In June, Rep. Edward J. Markey, D-Mass., sent a letter to Facebook CEO Mark Zuckerberg urging caution as the tech giant stakes out plans to open to children.
"We believe strongly that children and their personal information should not be viewed as a commodity to be bought and sold to the highest bidder," Markey wrote in June.
The Federal Trade Commission is also attempting to strengthen protections for children. Last week, the agency proposed stronger child privacy rules that would prevent advertisers from collecting information about those 12 and younger without a parent's permission.
The online market aimed at children is lucrative, largely because it is untapped, investors say. Some analysts and companies such as Kazaana estimate that tweens -- children between the ages of 8 and 14 -- spend $43 billion a year, by doling it out themselves or influencing their parents' buying habits.
"These are users who aren't locked into anything right now that is safe," said Pete Thomas, a partner at Silicon Valley-based ATA Ventures, which funded Kazaana. "There is tremendous opportunity to create a platform that pulls them in."
Some social-network advertisers are moving beyond the traditional banner ad. Kazaana hopes to get companies such as Gap to pay for users to dress up their avatars in the clothes maker's jeans and khakis. Proctor & Gamble's BeingGirl.com and its fan page on Facebook have attracted preteen girls who can find advice on menstruation and talk to peers.
Even with these efforts, child-focused social networks are confronting the same financial challenge befuddling Facebook -- how to survive as a business. Facebook has said it is having trouble making money off users who access the social network on smartphones and tablets, which are the favored platforms of youths.
Making subscribers loyal will be a challenge, analysts say. Many early adopters of new social networks try multiple services at once. About 20 percent of Yoursphere users also have Facebook accounts.
Still, advertisers say the new social networks have the potential to influence the spending habits of children and their families. If Abercrombie & Fitch sponsors a T-shirt design contest on Kazaana, they get children, as well as parents, to think about the brand.
"We have a whiteboard with, like, 50 ways to monetize, and we envision the relationships -- moms, grandparents and other family members on the site -- as a big part of it," said Michael Adair, Kazaana co-founder.