Image Source: Instagram.Instagram is going after a piece of the $9.9 billion digital video advertising market. As of February, the amount of time its users spend watching video had increased 40% over the preceding six months, according to Facebook (NASDAQ: FB) COO Sheryl Sandberg. And advertisers are noticing.Brand Networks, one of Instagram's early ad partners, found that 58% of ad impressions for its clients were video ads.