The Chainsmokers, composed of Drew Taggart and Alex Pall, had noticed how the word selfie had become a trend and they wanted to take advantage of it. They made a demo of a song containing monologue from a female clubber about taking good selfies. Inspiration came from the regular club going women that the Chainsmokers saw on their nights out in New York City. So when they were composing the track, they wanted to incorporate the "let me take a selfie" line in it somehow. The Chainsmokers described in an interview with The Phoenix New Times,

Honestly, "#Selfie" is kind of a phenomenon for us. We made it and thought it was funny and put it out as an edit. Then Dim Mak wanted to buy it, and they bought it from us and put it out officially and we made a video for it. This was all just because we thought it was fun. And then the song just kind of took on a life of its own. Which is honestly really great because it's brought in a new audience of people that listen to music that we've made that we're more passionate about.

The duo released the track to their SoundCloud account along with other posts in Vine and Instagram.

A music video for the song was also released and features actual selfies from Steve Aoki, founder of Dim Mak, Snoop Dogg, David Hasselhoff, WWE superstar Tyler Breeze, Canadian DJ A-Trak, Darth Vader, and Ian Somerhalder among others.Oliver Luckett, CEO of social media marketing firm TheAudience, said that the song is "so on point – it's shit-white-girls-say-meets-a-club-remix". TheAudience introduced the Chainsmokers to Aoki, helped create the "#Selfie" video and released it on January 29, 2014, on YouTube, utilizing the network of celebrities to promote it. According to Pall, "the video inherently had this viral concept built into it, which certainly helped get it off the ground".

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Wikipedia