Nobody can argue that 2009 hasn't been a huge year for Twitter - the year that the microblogging platform truly went mainstream and showed the world the variety of things that Twitter's revolutionary communications medium is capable of. There were any number of big Twitter events this year, but five stand out as noteworthy because of the unique aspects of Twitter that they displayed and the effect they had on the world's perception of the microblogging platform.

Five Ways Twitter Changed the World in 2009

---

1. District 9 and Paranormal Activity Shoot to the Top

In a year that saw a number of big-name films released, two under-the-radar pictures got major critical acclaim and, eventually, major media coverage - the science fiction docu-thriller District 9 and the supernatural horror flick Paranormal Activity. The two movies, both filmed and produced on low budgets, eventually shot to the top of the charts - and Twitter played a large part in that. The two movies were the third and fourth most-discussed movies on Twitter, just under New Moon (the second Twilight movie) and Harry Potter - both huge blockbuster hits with massive budgets. The popularity of District 9 and Paranormal activity can largely be attributed to person-to-person recommendations, and Twitter showed the world in 2009 that recommendations are one of the things it's best at.

2. The Hudson River Plane Crash

When a US Airways plane with 155 people on board ditched into the Hudson in January, the news was broke by a mobile phone and Twitter. The spectacular crash, in which no passengers were killed, was initially reported on by witnesses at the scene who tweeted photographs and status updates as they made their way toward the ferry to help rescue the passengers. The event firmly established Twitter as the network of choice for so-called "citizen journalists."

3. Michael Jackson's Death

Michael Jackson's death in June was the biggest news around the world for days; that news was spearheaded by Hollywood gossip mogul TMZ with a single tweet breaking the news of Jackson's death. Within minutes, word of Jackson's death had spread throughout Twitter, circling around the world several times over; Twitter eventually crashed under the burden of so many users tweeting about the event, which actually isn't all that remarkable, considering that Twitter crashed a number of other times in 2009 for less discernible reasons. Regardless, news of MJ's death being initially circulated via Twitter showed the world that Twitter's power as a tool of information (or, potentially, misinformation) about real-time events for which no official information exists - a theme that would persist throughout 2009.

4. The Fort Hood Massacre

When a gunman opened fire on Fort Hood military post in Texas, citizen journalists were again there to document the fast-moving story; this time, however, those citizen journalists came under fire from both bloggers and the mainstream media for reporting on the crisis instead of helping the victims. One Twitter user who tweeted a photograph of one of the shooting victims was identified and vilified by TechCrunch bloggers and commentators, who called her tweets inhuman and lacking compassion. The media also criticized Twitter citizen journalists for initially spreading misinformation about the shootings in the absence of official information. In the long-run, the tragedy may well be remembered on Twitter as the moment that Twitter users began wonder about how their behavior on Twitter affects them and the people around them.

5. #IranElection

Iran's disputed presidential elections were world news; every country in the world, it seemed, watched with baited breath to see how Iran would fare as discontented supporters of the defeated presidential challengers took to the streets to protest the elections, which much of the world considers to have been unfairly rigged in favor of the incumbent president. Supporters of the defeated hopefuls took to Twitter to organize the protests, distributing information in real-time about the locations of police and authorities.

The protesting, in many places, erupted into violence, and images and videos of the violence were shared via Twitter with the rest of the world. At one point, the US government actually contacted Twitter to ask them to delay a scheduled downtime which would have come during the busiest part of the day in Tehran, saying that they needed Twitter to remain viable as a communications channel with Tehran. The Iranian government eventually infiltrated Twitter, spreading misinformation and monitoring the activities of protesters through Twitter; when protesters and supporters around the world countered by changing their Twitter locations to "Tehran, Iran" to confuse the Iranian authorities, the government there simply blocked Twitter outright.

Although in the end, the protesters were unsuccessful at ousting the incumbent Iranian president, they did manage to make the rest of the world aware of the fact that much of Iran does not support its leadership, and that Twitter is a powerful communications channel during times of crisis, and especially when traditional channels of official communication cannot be accessed or cannot be trusted.

---

Overall, 2009 was a breakout year for Twitter; with that in mind, we're looking forward to next year, and trying to imagine what this list is going to look like in twelve month's time.