Interestingly, Vine downloads seem to already have started sliding in several key markets just a few days after Instagram began attacking it directly. Last Thursday, Instagram added a notably Vine-like video recording feature that was met with plenty of interest from users but still put Wall Street to sleep. The popular, Facebook-owned photo-sharing service now offers the ability to shoot and share 15-second video clips, mimicking the 6-second clip-sharing service that Vine pioneered last winter. Some industry observers view the Instagram attack as potentially fatal to Vine, while others see it as a lame copycat move unlikely to hurt Vine’s formidable momentum. According to Onavo, Vine is one of the most successful mobile apps in America in recent history, converting over 10% of American iPhone owners into active Vine users in just half a year.