For the record Seb, 'Fireflies' was #1 in the US billboard for a few weeks, is still in the Top 3, and is one of the biggest selling songs of the year.
It's also had 17 million plays on YouTube, which is very impressive, considering that the Black Eyed Peas song 'I Gotta Feeling' has had the most radio play of the year and was #1 on Billboard for something like 12 weeks, and only has 16 million YouTube plays. Both videos are the official uploads hosted by Universal Records.
So it could be said that from a popularity point of view, 'Fireflies' is the biggest song of the year. I was just in the US for 2 weeks myself and it was playing all the time on the radio, I heard it more than any other song, although the Jason Derulo song was a close second.