Facebook, Twitter and other social media have allowed Egyptians, Iranians, Chinese and others to challenge repressive governments. The governments, in turn, have been criticized for shutting the media down. Now countries with traditions of protecting free speech face a different challenge. Recent incidents, from London to San Francisco, have led to crackdowns, including prior restraint in the name of law and order. When social media are used to advocate violence, organize riots or coordinate crime, can government block access to the perpetrators and others as well? Will there be new limitations on the right to free speech as the law catches up with the technology?