If you suspect online "stalking" or sexual exploitation of a child, report it to your local law-enforcement agency. The National Center for Missing & Exploited Children (NCMEC) has a system for identifying online predators and child pornographers and contributing to law-enforcement investigations. It's called the CyberTipline®. Leads forwarded to the site will be acknowledged and shared with the appropriate law-enforcement agency for investigation.
Source: Netsmartz.org