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Social networking has always been a pupil magnet, and I'm not talking about eyeballs. Facebook was originally launched as a site where college kids could share personal snapshots. It was a walled community; only university students with .edu e-mail addresses could register. Playboy (NYSE:PLA) is using a similar approach in PlayboyU, last month's launch of a non-nude social networking site that is inviting only college students to participate.

News Corp.'s (NYSE:NWS) MySpace has always had a more liberal admissions policy, yet it too remains a hotbed for high school and college students. With roughly 200 million registered users, MySpace has a little bit of everything on its site. Many users are campus kids who would rather give out their MySpace profile page URL than surrender their cell phone number.

Social networking knows its audience well, so it's no surprise to see many of these sites getting excited about the new school year. Traffic tends to taper off during the summer at some of the more popular social networking sites.

Whether the "friends" are real or virtual, sharing photographs and snappy comments on customized profile pages has become a rite of passage for students. It's big business, too. Google (NASDAQ:GOOG) pays MySpace at least $300 million a year as its ad-serving partner. Google also owns its own social networking website -- Orkut -- that may not be a household word domestically, but it's insanely popular in certain foreign markets like Brazil.

It's easy to see why everyone from TV network executives to video game makers are wary of social networking. When kids spend time online, they don't have a lot of time for other leisurely pursuits, especially when school and homework consume so much of the day.

That's not the only thing that foes of social networking worry about. The biggest trend in social networking has been recent launches of several avatar-driven communities.

Sure, sites like Viacom's (NYSE:VIA) Neopets have done this for years. But this year has seen the launch of a Bratz-themed community and Mattel's (NYSE:MAT) BarbieGirls.com. Not to be outdone, Disney (NYSE:DIS) announced the acquisition of Club Penguin, just as it's in the process of beta-testing a Pirates of the Caribbean-themed animated community site.

The wave of toy-based social networking sites is intriguing because it aims for a crowd that's typically too young to register for accounts on the less-censored social networking sites. MySpace, for instance, will boot any user if caught lying about being 14 or older.

These graphically intensive social sites are either free or subscription-based, and most offer options to purchase or acquire virtual accessories. They are also very popular. Barbie Girls signed up 3 million users in its first two months without an active marketing campaign.

As you see, it's not only high school and college students having cyber-social fun these days. Elementary school kids are also hooked on Web interaction, no doubt training themselves for graduation into more conventional social networking.

Teachers? You have your work cut out for you this year as you face a classroom of students who may be more in tune with virtual responsibilities than real ones.

This summer's biggest hits in social networking:

Disney is a recommendation in the Motley Fool Stock Advisor newsletter service. Mattel is an opportunistic pick in Inside Value. Playboy is a sultry selection for Rule Breakers subscribers. Want to improve your stock-picking skills? Grab 30 days of free lessons with trial subscription offers to any of these newsletters.

Longtime Fool contributor Rick Munarriz remembers when social networking took effort. He owns shares in Disney. He is also part of the Rule Breakers newsletter research team, seeking out tomorrow's ultimate growth stocks a day early. The Fool has a nonvirtual disclosure policy.