US citizens' online political participation is on the rise, fueled by the phenomenal growth of social media. In the 2008 election, the Pew Internet & American Life Project found that, for the first time, the majority of US adults (55%) used the Internet to stay informed or get involved in the political process (Smith, 2009). Two years later, the Pew Internet & American Life Project reported that, in the 2010 midterm elections, 73% of adult Internet users (54% of US adults) received political news or information, or got involved online, and 35% of social networking site (SNS) users (about 22% of online adults) visited these sites for political information or getting involved in the campaign (Smith, 2011a, 2011b). A more recent Pew survey found that, on SNS, 66% of social media users (39% of US adults) engaged in one of eight civic or political activities (Rainie, Smith, Schlozman, Brady, & Verba, 2012). Political scientists and communication scholars urged politicians to tap the huge potential of engaging voters and campaigning via social media (Utz, 2009) because those who use social media for political purposes are more likely to vote or to donate to a campaign (e.g., Kim & Geidner, 2008). Political candidates have turned the Internet into a major fund-raising tool since 2000 when John McCain made history by raising US$2.7 million within 3 days of winning the New Hampshire primary (van Natta, 2000). In the 2008 presidential campaign, Barack Obama's campaign raised US$500 million dollars of contributions with the help of 35,000 groups organized by My Barack Obama website, the most popular Facebook page, and 1,800 YouTube videos (Learmonth, 2009). Of US$1.1 billion raised by the Obama team in the 2012 presidential campaign, US$690 million came from online donations (Green, 2012; Mason & Tanfani, 2012). Little is known about how US millennials' general social media use, political use of social media, political self-efficacy, general social trust, and online social capital influence their online political participation, although there is a growing body of research literature on social media use and political participation since 2004-the birth year of Facebook. Most of them attempted to discover whether the general use of social media could enhance young people's online and offline political participation and civic engagement (e.g.,