For years, the advertising business has been driven by the Madison Avenue mythology of small independent shops coming up with the snappy catchphrase or memorable TV commercial that becomes part of everyday culture.

But the announcement on Sunday of the merger of two industry giants, Omnicom and Publicis, to create the largest ad company in the world, signals that advertising is now firmly in the business of Big Data: collecting and selling the personal information of millions of consumers.

That business is a competitive one, with technology companies like Google and Facebook using their huge repositories of user data to place ads. Between them, Omnicom and Publicis accounted for $22.7 billion in revenue last year, more than the next highest ad firm, WPP. But no ad company comes close to the $50 billion in revenue that Google made last year, largely on the strength of its advertising business.

The merger was announced in Paris. There, the chief executive of Publicis, Maurice Lévy, said the “billions of people” who are now online and providing data to companies offer an opportunity to use advertising technologies to crunch billions of pieces of data “in order to come with a message which is relevant to a very narrow audience.”