According to Official Guinness Records,
The YouTube broadcast of Prince William’s marriage to Catherine Middleton (both UK) in London, UK, on 29 April 2011 achieved a record 72 million live views, as people from 188 countries around the world tuned in to watch the event on the company’s official Royal Channel. Although this figure alone was enough to beat the 70 million streams achieved during the inauguration of US President Barack Obama in 2009, the wedding’s overall tally is likely to have been significantly higher when taking into account the millions watching via other live streaming services.
For a complete list of 2011 records, please visit 2011 Guinness Records in United Kingdom.