Apparently, a reform candidate was better at using Facebook than the status quo candidate. So the government shut the whole thing down. This is the sort of stupid thing that repressive governments do when they feel threatened by new technology. (And, by the way, repressive regimes are always threatened by new technology, especially the kind that lets people communicate with each other).
It may work for now. Mir Houssein Mousavi may lose the election. Maybe he would have anyway; who knows? But the Iranian leadership has just surrendered any plausible claim to openness in its electoral process. Worse yet (or better, from our perspective) it has done so in a way guaranteed to piss off young people. (And remember that fully half the Iranian population is under 24 years old, versus less than 6% over 65).