Last fall, Facebook helped get out the vote on election day in the United States with a special home page promotion in which over 2 million people shared with their friends that they’d voted on Facebook. The rally helped spread messages to millions more Americans that encouraged them to take part in US national elections themselves.

Today, Facebook is doing the same thing on national election day in Iran. Facebook is showing a home page message to all users in Iran which says, “Today is Election Day in Iran” with an “I voted” button and a counter.  Clicking “I voted” will increment the counter and pop open a form that gives users the option to share a message with their friends that indicates they voted, giving the voting message viral distribution.

So far, over 12,400 supporters of both Mahmoud Ahmadinejad and his chief rival, Mir Hossain Moussavi, have participated. Facebook says it’s likely to do similar campaigns in other countries for future elections. No word yet on what percentage of Facebook users are advocating for Ahmadinejad vs. Moussavi, though pro-Moussavi groups and pages have had more members to date.

“Facebook’s features for connecting and communicating naturally support political discussions and organizing.  We think the democratic process is important so we’re making special efforts to foster participation,” Facebook’s Barry Schnitt says. “We’ve done this in the US and now in Iran.  We hope to do the same thing for more elections going forward.”

Facebook’s United States election day promotion on November 4, 2008: