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Then in 1989 the World Wide Web was created, based on the "hypertext markup language" invented by physicist Tim J. Berners-Lee. He made it easy for millions of techno-neophytes to get on the Web. In 1993 a programmer in Finland invented one of the first anonymous remailers, letting a user obfuscate the source of messages he posted to a forum.

"People used it to exercise grudges," says Eugene H. Spafford, a Purdue University professor heavily involved in Usenet at the time. When he intervened with a Finland Web host on behalf of a researcher who had come under masked attack online, many of his Usenet peers turned on him. He got dozens of e-mails accusing him of censorship and received two death threats. "I found it ironic that people professing a concern for free speech were threatening to shut me up forever," he says. He resigned from his Usenet role in 1993 amid "rising incivility," he says. "Freedom of speech without any cost can be freedom from thought."

Then came the rise of anonymous chat rooms on America Online in 1994; users could veil themselves in up to five onscreen nicknames. "That was the difference between survival and bankruptcy" for AOL, says David Siminoff, an early investor and now a partner at Venrock. "The gay community came out on AOL, and anonymity was the alcohol in the drinks."

Congress added a splash of impunity to the mix in 1996, when it passed a pro-Internet law known as the Communications Decency Act. Among other provisions the law insulates electronic middlemen from libel suits. The Web site that passively furnishes a vehicle for online postings is deemed to be no more responsible for the resulting content than a phone company is for any criminal conversations that take place over its network.

The effects of masked behavior online grew darker with the rise of blogs (the first one is said to have debuted in late 1997, and a decade later some 70 million exist). It accelerated more recently with the explosion in social network sites such as MySpace (with 110 million users). Since 1998 Icann, a policy-making group, has controlled domain registration on the Net, logging new sites in an open database called Whois, but the data isn't verified, and even gobbledygook triggers no alarms.

By the mid-1990s new businesses emerged to ensure anonymity on the Web. One early entry, Anonymizer in San Diego, now provides service at $30 to $50 a month to 100,000 customers who surf in secret via 20,000 Net addresses. When a subpoena is served, Anonymizer complies, but "we have no information to divulge," says founder Lance Cottrell. He doesn't store any data on where clients go online. He has no qualms: "I feel the same way I would if I worked at the U.S. Postal Service and people were dropping off ransom notes in my mailboxes. It is such a small fraction of what's going on," he says.

But using anonymity to shield the powerless from oppression and ostracism is vastly different from letting faceless attackers run amok without fear of the reprisals and penalties that would punish them in the real world. In some quarters free expression online could become a tradeoff: Say anything you like, so long as we know who really is saying it. The pendulum of anonymity has begun to swing toward new kinds of restraint.

"Bad citizens will be marginalized on the Web," vows Richard Palmer, a security chief at Cisco Systems (nasdaq: CSCO - news - people ). It has a thousand engineers overhauling routers to better discriminate between "good" behavior and "bad" behavior based on constantly changing reputation rankings of computers on the Web, though others say making this work will be all but impossible.

Facebook, with 42 million users, is less wild than its forerunner MySpace, now a unit of News Corp. Facebook limits interactions, based on member behavior; each user has a score in a system called Karma. Your score goes down if you flirt up ("poke") too many users or if you are over 18 and e-mail a lot of kids.

The privately held site blocks some accounts for suspicious activity (it won't say how many). "We knew this would cause our site to grow more slowly, but we think the world is moving in our direction," says Facebook privacy chief Christopher Kelly. Some smaller sites set up online gated communities. At BlogHer, a female blog network run out of Redwood City, Calif., 60 editors vet postings to stamp out abuse, harassment, stalking and threats.

None of this is moving fast enough for the Catsourases as they mourn Nikki's death and its online aftermath. They spent $3,000 to hire ReputationDefender of Menlo Park, Calif. to methodically scrub the Web of the ghastly photos, contacting each site and pleading and pressuring. MySpace, Photobucket and most other outlets agreed. Founder Michael Fertik says business is good: 2,000 clients in his first year, sales at $2 million and rising. "Anonymity on the Web turns too many regular people into beasts," he says. Lesli Catsouras says: "I don't believe in anonymity, not in this country. We're too privileged to allow this to happen. Everyone will get hurt by this. Just watch."

Editor's note: The print version of this story stated that the Catsouras family paid $20,000 to ReputationDefender. That figure represents ReputationDefender's estimate of the cost of clean up. The firm did not charge for much of the work. Aditionally, we stated that Katherine Sierra asked Meankids to remove the content. She did not.