By Jia Lynn Yang, writer

During protests in Iran this summer over the re-election of President Mahmoud Ahmadinejad, social networking tool Twitter’s raison d’être overnight went from frivolous to vital: The world outside Iran followed every spurt of  information that trickled out on mobile phones outfitted with the Twitter application.

Since then, activists have only grown more sophisticated in how they organize protests and spread information online. These days all the action—inside the country and among politically active émigrés—is on Facebook and a Digg-like site called Balatarin.com.

Nikahang Kowsar, an Iranian political cartoonist based in Toronto who also runs an Iranian news hub, has more than 11,000 friend connections on Facebook. The limit per profile is 5,000, and so Kowsar runs three profiles, spending 11 hours online a day, responding to emails, running his news site and maintaining his profiles.

Iranian journalist Masih Alinejad, who’s been living in exile in London for two years, also maintains three profiles. Alinejad says she asked Facebook to lift the limit, arguing her profile was a tool for political protest. She says Facebook denied the request.

How protesters protect themselves on Facebook

The Los Angeles-based founder of Balatarin, Mehdi Yahyanejad, attributes Twitter’s rise last summer to the fact that it was the only English-language source of information during the first few weeks after the disputed June 12 presidential elections. And so the Western media instantly gravitated towards it.

But, in fact, other social media are more popular among Iranians, especially educated, middle-class folks who support opposition leader Mir-Hossein Mousavi.  “In Iran people use Facebook,” says Yahyanejad.

Iranians also like Yahyanejad’s Balatarin, a community site that helps users find online news and information about Iran and issues of interest to Iranians around the world. Though Balatarin (the name means “highest” in Persian) officially has been blocked inside Iran for three years, politically and tech savvy Iranians have found ways to get to the site.

To protect their anonymity, some politically active Iranians still inside the country adopt pseudonyms, such as changing their last names to “Irani.” The fear is that the government will use Facebook as a tool for spying on its citizens. Journalist Alinejad says that when one of her friends in Iran was arrested, her brother sent a message to her through Facebook saying he was removing Alinejad from his sister’s profile, so the government couldn’t use the connection to incriminate her.

Balatarin’s Yahyanejad and others also said that people entering Iran have been stopped at airports and asked about their Facebook profiles. As a result, there is advice spreading that people take down their profiles before entering the country and then reactivate them after they leave.

Accounts suggest the site is blocked inside the country, although protesters can use software to get around the filters.  Facebook told FORTUNE it could not release the number of users in Iran, or confirm whether the site was being blocked.

Alinejad recently organized a whole campaign from London using just Balatarin, which focuses on political news coming out of Iran, and Facebook. In the most recent wave of demonstrations, centered around National Student Day last week, the Iranian government apparently arrested Majid Tavakoli, a student leader, and forced him to go on TV wearing a hijab, traditionally an item of women’s clothing. Alinejad wrote a post on her blog suggesting that other men wear hijabs too in solidarity. A link to her site wound up on Balatarin, where a debate ensued, and soon the campaign moved to Facebook, where men began posting pictures of themselves wearing hijabs. Alinejad says more than 500 people have sent photos as part of the campaign.

Says Alinejad, “[The government] thinks if they arrest journalists, they can stop news from spreading around the world, but they can’t.”