In one sudden leap, social networks have been transformed from trivial pop-culture fad to a grass-roots communication medium worthy of being stifled by government.

Al Jazeera is reporting that Iranian authorities have begun filtering out Internet traffic for Twitter and Facebook, along with jamming Farsi-language radio newscasts by the BBC and The Voice of America.

Meanwhile, Twitter says it will delay a planned Monday night upgrade to avoid cutting daytime service to Iranians who have been using it to coordinate protests against the disputed re-election of President Mahmoud Ahmadinejad.

Tweets from the streets of Tehran have been important to Al Garman, an Iranian expatriate living in Bellevue, Wash., who has been glued to his Blackberry, according to this story in the Seattle Times. "Many of the tweets are ahead of the news organizations. The stories in the news organizations come after the tweets," Garman tells the Times.

Iran's government has not commented on the restrictions, instead accusing the international media of exaggerating the reports of violence against protesters, and conspiring to destabilize the government.

Update: It actually was the U.S. State Department that asked Twitter to postpone its scheduled maintenance so its services could be available to protestors and activists on the ground in Tehran, according to this Washington Times story. That move worked in concert with a contribution from San Francisco technologist Austin Heap, who posted these instructions for how to use proxy servers to bypass government Twitter filters.

Also, tech savvy protestors can just as easily reach Twitter using proxy tools readily supplied for free at proxy.org, notes Chris King, director of product marketing at Palo Alto Networks, a network security firm.

"It's a bit of a cat and mouse game," says King. "The government will discover and close these proxies. But the sophisticated activists will just move to the next one. They can get through any blocks put up by the government."