A few months before the election, Facebook became available in Iran.
More tech-savvy Iranians (armed with proxies) were already using FB.
But once the social networking site was not filtered in Iran, there was an avalanche of Iranians on FB. I suddenly started to hear from classmates I hadn't heard from in more than 20 years. It was pretty amazing. They all seemed to quickly master the many ways in which social networking could be used to advantage. And so, yes, it was used very effectively to get videos and messages out from the various campaigns, especially (opposition candidate Mir Hossein) Mousavi's campaign in the lead-up to the election. But this was more the case before the information blackout.
It's rare to see someone from Iran on FB now. Many have taken down their profiles or changed their names. So I don't believe it's any longer a primary source of information in the shape of protests.
To the best of my knowledge, Twitter was not initially blocked, even after some of the other popular sites were. Maybe it wasn't that well known in Iran then, or perhaps it didn't initially appear to be a threat. You're limited to 140 characters and you can't exactly use it as a platform to upload photos and video. But that turned out to be its very charm. At a time of crisis, it was like a telex machine. At Tehran Bureau, we turned to Twitter when our Web site was hacked. I started to put out paragraphs a sentence, sometimes a half sentence, at a time. Bloggers and journalists following us stitched these together and began to quote us in their blogs and publications. This expanded our audience very quickly.