Washington - When Americans voted to renew US President Barack Obama's lease on the White House in November 2012, more than nine million citizens took to Facebook to click the "I'm a Voter" button, showing their online friends that they had cast a ballot.The social media site plans to offer versions of the feature around the world for a slate of national elections this year, the company said on Monday.The feature was available for voters in India as the world's largest democracy chose a new prime minister, Narendra Modi, in voting over recent weeks. Over four million Indian voters used the "I'm a Voter" button during the country's parliamentary elections, Facebook said.For Facebook, this is another effort to integrate its services into the everyday life of people around the globe as it seeks to increase its number of users, particularly in emerging markets.The button will appear for voters in next week's European Parliament and Colombian elections, and for citizens in South Korea, Indonesia, Sweden, Scotland, New Zealand, and Brazil later this year. It will also appear again for Americans, during November's midterm congressional elections.'Social multiplier effect'By clicking the button, users broadcast their status as a voter to their network of friends, but do not reveal how their vote was cast.Previewing the feature's worldwide roll-out, Facebook said it estimates nearly 400 million people will see the message in their news feeds this year - more than one third of its roughly 1.1 billion active users.A 2012 study in scientific journal Nature found that in the United States' 2010 midterm elections, 340 000 additional citizens voted after seeing that their friends had clicked the button to indicate they had cast a ballot.That election heralded the rise of the conservative Tea Party movement for smaller government, as Republicans swept into office and claimed the US House of Representatives from Democrats."There is a real social multiplier effect," said Andy Stone, a spokesperson in Facebook's Washington office. "When people see on Facebook that their friends have voted, they themselves are motivated to vote."