IN 2015, FRIENDSHIP WAS THE BIGGEST TREND IN FASHION

Almost a year ago to the day, Taylor Swift threw herself a 25th birthday party. Many of Swift's famous friends were there, including (but not limited to) Sam Smith, Selena Gomez and Justin Timberlake.

aIt was all business as usual untilBeyoncé and Jay Z were spotted lurking in the background of a group picture. The Internet lost its mind.

It didn't take long before Swift's "#squadgoals" phenomenon began captivating the same portion of cyberspace that was once flummoxed as to how she and hip-hop's greatest power couple became acquaintances. Soon, Swift's friendships within the fashion industry — Gigi Hadid, Martha Hunt, Lily Aldridge, Karlie Kloss and more — began taking over our collective Instagram feeds.

While fashion has long been insular, a portion of its exclusivity hit a fever pitch in 2015. Despite social media opening up this world to any human with access to a computer or smartphone, the you-can't-sit-with-us mentality remained prevalent. No longer restrained to the likes of Anna Wintour and Karl Lagerfeld, cliquey friendships seemed to resonate with models, designers and "It" girls more prevalently than in 2014. 

Much of this can certainly be attributed to Olivier Rousteing, whose own "Balmain Army" has helped to create a rabid enthusiasm for the brand. As the first French house to surpass the million-follower mark on Instagram,Balmain has made a business of casting entire groups of friends in both runway shows and campaigns. Rousteing sits at the head of the table as the crew's common thread.

The 30-year-old creative director has as a knack for recruiting talent as well as for growing Balmain's digital and retail fan bases. In October, the house debuted a capsule collection for H&M with a bumping runway show — Kloss,Kendall Jenner, Gigi and Bella Hadid, Joan Smalls, Jourdan Dunn andAlessandra Ambrosio all walked — followed by a rare performance by the Backstreet Boys. That "interest" in the line went on to "[exceed] all previous collaborations" was no surprise, even despite the range's steeper price point. It literally paid for Rousteing to have friends in high places.