In some limited circumstances, a website may be liable for the conduct of its users. However, in most instances, a website will not be liable for things posted on a site or to actively seek to verify the ages of users. As you might imagine, this would put a very heavy burden on websites and it could actually make it difficult or impossible to operate the sites. Considering the scope of many major user-driven sites, you can imagine how costly and difficult such a burden could prove.
Generally, the burden of making sure a child isn't engaging in inappropriate activity online falls on the parents. However, again, it's worth emphasizing that the nature of a website can alter that responsibility and a case could potentially be made against a website that has permitted a child to engage in conduct that the child should not be involved in on the site. This is especially true when the child's age should be apparent, could readily be evaluated, has been reported previously, etc.
conduct takes place on a website, such as improper communications or conduct with a minor, the parties to that conduct can be charged with a crime. If an individual wishes to see criminal charges pursued, he/she can report the matter to law enforcement. With a website, reporting the matter to federal officials (such as the IC3: http://www.ic3.gov/default.aspx) can be a good bet, as internet crime is generally the domain of both state and federal authorities. For this purpose, a victim does not need to retain an attorney. Instead, the government handles the prosecution and the investigatory side of the matter.
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