Nicki Minaj's 'Starships' remains at two, followed by Carly Rae Jepsen's 'Call Me Maybe', which leaps from 15 to three.
Meanwhile, Skrillex climbs from five to four with 'Bangarang' and Chris Brown's 'Turn Up The Music' completes the Top 10.
Further down the chart, Labrinth and Tinie Tempah's 'Earthquake' vaults from 21 to 11, Ed Sheeran's 'Lego House' zooms from 49 to 19, and Far*East Movement and Justin Bieber's 'Live My Life' shifts from 28 to 20.
Outasight bounds from 37 to 29 with 'Tonight is the Night', and Kelly Clarkson's 'Stronger (What Doesn't Kill You)' re-enters at 39.
The Top 10 singles in full (click where possible for our reviews):
1. (1) fun. ft. Janelle Monae: 'We Are Young'
2. (2) Nicki Minaj: 'Starships'
3. (15) Carly Rae Jepsen: 'Call Me Maybe'
4. (5) Skirllex: 'Bangarang'
5. (3) Flo Rida ft. Sia: 'Wild Ones'
6. (4) Gym Class Heroes ft. Neon Hitch: 'Ass Back Home'
7. (7) Qwote ft. Pitbull: 'Throw Your Hands Up'
8. (6) 360 ft. Gossling: 'Boys Like You'
9. (8) Jason Derulo: 'Fight For You'
10. (9) Chris Brown: 'Turn Up The Music'
Watch fun.'s 'We Are Young' music video below: