Gangnam Style by Psy is the first video to hit one billion views on YouTube

"A million views? You know what's cool? A billion views.
Today, a 34-year-old K-Pop artist made online video history when his viral video, Gangnam Style, smashed our records and became the first video ever to reach one billion views. Yup, that’s right one BILLION views!
PSY's success is a great testament to the universal appeal of catchy music-- and er, great equine dance moves. In the past, music distribution was mostly regional. It was more difficult to learn about great artists from around the world. But with a global platform at their fingertips, people are now discovering and sharing amazing music from all over the planet, by artists like Brazilian Michel Teló and Belgian-Australian Gotye.
One billion views is an incredible number, but the PSY-nomenon goes beyond that. Check out these stats:
PSY was already big in Korea, but in 2012, he became a global celeb as Gangnam Style quickly spread from Seoul and the pacific to North America, South America, and Europe. It’s been seen at least 1 million times in close to 75 countries, making it one of the most global music sensations ever!
From a one-thousand person flash mob in Jakarta to cover videos from Ai Weiwei and Mitt Romney, hundreds of thousands of parodies have been uploaded to YouTube, some of which have tens of millions of views. In fact, fan tributes to Gangnam Style are now being viewed 20 million times every single day.
PSY's own remix with Hyuna has 200 million views alone.
"Gangnam Style" was YouTube's top rising search of 2012 and on October 6th, we saw more than five million searches for “gangnam style” in a single day. Check out this video demonstrating some of our most popular YouTube searches this year.
For those interested in the business side: a number of assessments and projections have been posted claiming “Gangnam Style” has generated over $8.1 million in advertising deals, hit more than 2.9 million in song downloads since July, and achieved other incredible feats!
Since late last month, people have clicked to buy the track on iTunes over 600,000 times helping make PSY the first Korean artist ever to rank #1 on the U.S. iTunes chart and #1 in over 30 more countries.
Perhaps what’s most impressive about this feat is that it took just over five months to happen."

Source:  YouTube blog, 21st December 2012
Earlier - It became the most viewed video of all time in late November
Earlier - 100 million views in the first 50 days (5th September)