Google's share of US advertising could drop below 50% for the first time in more than 10 years, according to eMarketer.Dilara Irem Sancar/Anadolu via Getty ImagesGoogle is facing new competition in the advertising market as AI transforms the industry.Google's search ad market share in the US could fall below 50% for the first time in a decade.Perplexity AI, meanwhile, says it is attracting "top-tier" advertisers.Google is facing a challenge to its decadeslong dominance of the internet's advertising market as AI, the new kid on the block, changes everything.Projections by eMarketer — a digital marketing research company owned by Business Insider parent company Axel Springer — say that Google's share of the US search advertising market could drop below 50% next year for the first time in more than a decade.Long dominant, Google has begun to slip in other significant ways too.