In June 2010, the U.S. Department of Homeland Security confirmed publicly (PDF) that its agents were permitted to create accounts on social-networking sites in some situations.

DHS's National Operations Center "will monitor activities on the social-media sites" using search engines, aggregators, and other tools, last year's announcement said. "The NOC will gather, store, analyze, and disseminate relevant and appropriate de-identified information to federal, state, local, and foreign governments, and private sector partners..."

In addition, the Electronic Frontier Foundation unearthed documents showing that DHS officials were sending "friend" requests to people applying for U.S. citizenship. DHS conducted extensive monitoring of social networks during Obama's inauguration.

In 2009, CIA investment arm In-Q-Tel invested in Visible Technologies, which monitors millions of posts on social-networking Web sites, Wired reported. Tax collectors, too, are "nabbing scofflaws by mining information posted on social-networking Web sites," according to The Wall Street Journal, and the FBI has previously supported legislation that would allow federal police to monitor the Internet for "illegal activity."

This move toward monitoring social networks hasn't been without controversy. A New York Times editorial suggested these techniques may go too far: "If government agents are joining social networks under false pretenses to spy without a court order, for example, that might be crossing a line."

It's also not been limited to the United States.

In 2009, the U.K. Home Office announced it would monitor all conversations on social-networking sites, including Facebook, MySpace, Bebo, Twitter, and Skype, in a crackdown on terrorists' use of the Internet. So has the Chilean government. And, of course, some repressive regimes have simply blocked Web sites completely.

Update 4:20 p.m. PT: Here's some background from a House Homeland Security hearing on July 27, where Rep. Michael McCaul, a Texas Republican, warned of Internet radicalization:

We're investigating the radicalization of Muslim youth in the United States. Does anybody on this panel disagree with the notion that the radicalization of Muslim youth in the United States poses a threat to our homeland security? I take it by your silence that you agree with the idea that the radicalization of Muslim youth in the United States poses a direct threat to the security and safety of our homeland security. We know that three dozen Americans have left the United States, mostly from Minnesota, to join forces in Somalia, to receive training under al-Shabaab, to receive training by Al Qaida...

And clearly, al-Awlaki is becoming the emerging threat, you know, on the scene, in my judgment. He's radicalizing Muslim youth over the Internet here in the United States. And what easier way to do it? If you can't get into the country with travel documents, why not radicalize people who are already here? [Ed. Note: Anwar al-Awlaki is an alleged al Qaeda leader. Emphasis added.]

The same day, a subcommittee of the House Intelligence Committee held a hearing, where chairman Sue Myrick, a Republican from North Carolina, also stressed Internet radicalization. Referring to Samir Kahn, a U.S. blogger who reportedly moved to Yemen:

You know he was here, we knew it but we really couldn't do anything about it. Now, he's very successful because he's in a country where he can radicalized, he is radicalizing. We have proof of that with our young people. And you know, parents are very concerned about this happening to the young people because as you said the Internet, et cetera, is very easy today for people to get on any site they want to, and you know, be involved.

So, what is it that we can do? ... You know, what can we do and ensure that in the future not only with him but with others, how do we stop this or how can we continue to fight back against what could become homegrown terrorism, none of us want to see, and you know, that happens right here in our own backyard?