Under a 1998 law known as COPPA, for the Children's Online Privacy Protection Act, sites directed at those 12 and under must have verified parental consent before collecting data on children.
Some sites go much further: Disney's Club Penguin offers a choice of viewing either filtered chat that avoids blacklisted words or chats that contain only words that the company has pre-approved.
Yet even though defensive techniques are now available and effective they can be expensive.
They can also alienate some of a site's target audience - especially teen users who expect more freedom of expression.'There are companies out there that are doing a very good job, working within the confines of what they have available,' said Brooke Donahue, a supervisory special agent with an FBI team devoted to Internet predators and child pornography.
Facebook's software likewise depends on relationship analysis and archives of real chats that preceded sex assaults, Chief Security Officer Joe Sullivan told Reuters in the company's most expansive comments on the subject to date.