Because, simply, the opposition has been using the social networking site to get the word out about reformist candidates that could prove an embarrassment to the all-but-certain-to-get-another-term President Mahmoud Ahmadinejad. Specifically, reports the Financial Times, backers of Mir-Hossein Moussavi had been using the popular site to campaign. More:

“‘The filtering is certainly because of the election after the supporters of Mr Moussavi used Facebook as a channel to exchange views and got an upper hand compared to other candidates,’ said Behrad Beheshti, the head of a Moussavi election centre in Isfahan.

Mr Beheshti said there was software available to circumvent the filter, but added that the move was an indication that fundamentalist supporters of Mr Ahmadi-Nejad had felt ‘threatened’.

Reformists have complained that Mr Ahmadi-Nejad benefits from greater access to state TV. But the development of technology and its is adoption by a large number of people – 21m people out of a population of 70m have access to the internet – has made it more important in this election.

In the final weeks before the June 12 poll all candidates are guaranteed equal airtime.”