This question was recently put to Iranian Supreme Leader Ayatollah Ali Khamenei, reportedly by an Iranian via mail.
Ayatollah Khamenei's written response, posted on Iranian websites, was ambiguous:
"In general if it requires engaging in [immorality and evil acts] (such as spreading corruption, lies, and false materials) or if there is concern that it is sinful, or it strengthens the enemies of Islam and Muslims, it is not permissible. Otherwise it's fine," he wrote.
The fatwa is open to interpretation. One can decide, seemingly, whether by using the social-networking site he or she is committing a sin.
By issuing the fatwa, Khamenei has apparently contradicted the Iranian state policy of Internet filtering -- a policy that has been adopted and implemented with his blessing, as the supreme leader has the final say in all matters in the Islamic republic.
Iran has one of the world's toughest Internet censorship regimes, blocking millions of websites including pornographic sites, news websites, and social-networking sites such as Facebook.
Millions of Iranians who use antifiltering software to access Facebook don't necessarily think they're acting against Islam or spreading corruption. They use the social-networking site to stay in touch with friends, share photos and information, discuss political and nonpolitical issues, and also to join different campaigns.
Khamenei himself is on Twitter and some Iranian lawmakers are said to have Facebook pages. The Iranian government uses blogs and social media for its own purposes, including propaganda.
In recent years a number of Iranian officials have issued warnings and called on Iranians not to use social-networking sites, namely Facebook and Google+.
One of the latest warnings against Facebook came last week from Mehdi Jafari, who heads the technology and intelligence section of the Pupil's Basij militia. Jafari, who said there were currently 17 million Iranians on Facebook, warned against the "cultural damages" of cyberspace.
Shabestan, the religious news website that first reported about Khamenei's Facebook fatwa, says any use of social-networking sites helps the "enemies" by providing user data.
This view stems from claims by Tehran that Facebook and Twitter are being used by Western intelligence services to recruit agents and gather data on individuals.
Nevertheless, Shabestan concedes that the Khamenei's pronouncement is "in between."
With or without the fatwa, Facebook is among the top 10 most visited sites in the Islamic republic.
Ultimately, the episode demonstrates that the Iranian establishment can't simply ignore the popularity of social-networking sites, the fact that filtering is not completely blocking access to them, and warnings are not enough to keep Iranians logged off.
-- Golnaz Esfandiari