Adopt a notice-and-takedown procedure for copyright infringement claims
User-generated content (UGC) poses a range of legal challenges that never existed before the social Web. Fortunately, Congress came to the rescue in 1998 with the Digital Millennium Copyright Act. Site operators with a DMCA-compliant "notice and takedown" procedure in place enjoy immunity from lawsuits by content owners (such as record labels and movie studios) for infringing content uploaded by users. Without this "safe harbor" from liability, most UGC sites would be sued out of existence within months because of the enormous volume of materials uploaded by users and the difficulty in identifying which of them may be subject to copyright. To take advantage of the safe harbor, t's important to register a representative of your site as agent to receive DMCA takedown notices with the U.S. Copyright Office, to post the policy on your site, and to honor it in practice. See links below for further information.
Take care to keep children under 13 off the site
The Children's Online Privacy Protection Act (COPPA) sets strict limits on the use of private information gathered from children under 13 in the United States. Unless your service is designed specifically for children, the simplest approach for U.S.-based social media sites is to exclude children under 13 from the site. This can be done by asking for date of birth in the registration process; when a user is rejected for being under 13, the site should set a cookie so that the child can't simply back up and enter a different age to register. The alternatives are to either refrain from gathering any personal information (impractical for most sites) or require parental consent (impeding registration flow). For further information on COPPA compliance, see http://www.coppa.org/comply.htm.
Maintain a clear distinction between UGC and your own original content
The DMCA and a similar law (Section 230 of the Communications Decency Act of 1999) provide powerful protection against liability arising out of UGC. In general, UGC can be edited or selected by the company without losing the safe harbor protection, but material which is not user-generated doesn't qualify for the safe harbor. Accordingly, your original material should go through a careful review, fact-checking and rights clearance process before it goes on the site, in much the same way that a newspaper publisher or TV station would take those precautions. For small startups which lack the resources to go through these steps, the safest approach is to steer clear of any content which presents high risk for liability, such as content which accuses individuals or companies of serious wrongdoing or which borrows extensively from copyrighted works. When in doubt, consult your lawyer.