Twitter, Facebook, YouTube Navigate Islamic State Censorship | Civic | US News

Still, Silicon Valley companies have tread carefully on deciding when to restrict extremist content online because they're still figuring out how to address such an evolving, sensitive issue, says Emily Parker, author of the book, "Now I Know Who My Comrades Are: Voices From the Internet Underground."

But flagging posts in the name of national security also can be dangerous to political diversity, as the Egyptian government labeled its former opponents the Muslim Brotherhood a terrorist organization, while the U.S. has not done the same.

There are also concerns that governments in countries like India, Thailand and Vietnam have tried to remove posts from social media sites for political reasons, says Adrian Shahbaz, an Internet freedom researcher with the watchdog group Freedom House.