Several sources in Iran have told RFE/RL that Facebook and Twitter were freely accessible without the antifiltering tools that many Iranians use to access the blocked websites.
A young woman in northern Iran wrote on her Facebook page on September 17 that, “Facebook is not filtered?! Am I dreaming?"
Many users expressed joy over what they said was the social media site's sudden accessibility, while some said they still needed antifiltering software.
"Let's hope it lasts," one user told RFE/RL.
Facebook has been briefly unblocked in the Islamic republic before, presumably to let authorities update the regime's strict censoring software.
In recent weeks, several Iranian cabinet members have joined Facebook, sparking a debate over its blocking.
Facebook is the most popular social-networking site in Iran.
There was no immediate reaction from the government.