Two years ago, Twitter-owned Vine was growing like kudzu. It was widely embraced by both consumers and marketers who sought to push the envelope with creative six-second clips. But the explosion of new video formats on Facebook, Snapchat and YouTube, which all now boast sizable scale, has caused top brands to quietly slip away. "Over time, it became difficult for many marketers to achieve scale [on Vine]," explained Tyler Hissey, senior digital strategist at Hill Holliday. "In the last six months or so, brands have started to de-emphasize Vine as a channel because of the targeting capabilities on all these other platforms."