The luxury magazine market, for so long a well-heeled haven from the turmoil facing the rest of the print media industry, could be about to confront the same headwinds battering other magazines and newspapers. Analysts say that glossy magazines such as Vogue, Harper’s Bazaar and Vanity Fair are starting to see a shift in readers and advertisers to online social media platforms such as Instagram and Facebook. But they are also being hit by forces specific to the industry that has for so long offered them protection.