Turn is the biggest and fastest-growing of the major DSPs, or demand-side platforms bidding display audience buys for the major agency holding companies, according to new data being released by Standard Media Index. But the data also shows that exchange-based buying is a hyper-fragmented business that is actually dominated by a long-tail of suppliers. The data, which is compiled from the actual media buys processed by four of Madison Avenue’s six major holding companies -- Aegis, Havas, Interpublic and Publicis -- shows that “Other” is the biggest supplier of exchange-based buys, accounting for 80.4% of inventory purchased through exchanges during the first eight months of the year. It’s also the fastest-growing segment. Other’s share of the market expanded nearly 59% over the same period in 2012, suggesting that big agencies like working with a wide variety of exchange-based providers and solutions. Turn, the single biggest DSP, by comparison, accounted for only 7.4% of the market, though it’s also growing fast, with its share rising 20.5% over 2012. The next biggest DSP, MediaMath, had only a 1.5% share of the market, followed by DataXu’s 0.9% and x+1’s 0.6% share.