Too many new media companies are chasing too few advertising dollars, one of the advertising industry’s most senior executives has warned, as fresh questions were raised about the fast-rising valuations of prominent online groups.

“Everyone is seeing advertising as the manna,” said Maurice Lévy, chairman and chief executive of Publicis, one of the world’s largest marketing groups. “Far too many people are building plans based on advertising and they may well be disappointed because there is not enough money for everyone.”

Speaking at this weekend’s Monaco Media Forum, Mr Lévy likened the boom in businesses whose revenue models depend on the continued growth of online advertising to the dotcom boom and bust that began a decade ago.

“It’s exactly the same situation as we saw at the end of the 1990s, when everyone thought that because he had a website he’d get the valuation. Now everyone building a Web 2.0 operation believes he will receive the advertising.”

Although some social networking sites such as Facebook had succeeded, he said, the small amount of money they were making was still “unbalanced” compared with their large audiences because advertisers remained wary of being seen as intrusive.

“I’m not sure we’ve found the right way of communicating with that audience,” Mr Lévy said, but he expressed confidence that web advertising would continue to grow strongly, and at the expense of other media, even in an economic slowdown. This year Publicis bought Digitas, a specialist in online advertising, for $1.3bn.

His comments came as Barry Diller, who this week announced the five-way break-up of IAC, his US media and online conglomerate, challenged the $15bn valuation of Facebook implied by Microsoft’s $240m investment in the site.

“If it’s real money it’s insane,” he told the conference, adding that he believed instead that Microsoft had taken the stake to prevent Facebook from partnering with Google. “Let them sell another 99 per cent for $14.8bn and I’ll believe it.”

Such sites were fashionable “flavours”, Mr Diller said, pointing to the earlier market excitement about MySpace, the social networking site bought by News Corp. “The bloom is definitely off the MySpace rose,” he added.

Mr Lévy expressed confidence that marketing services groups such as his would not be challenged for such accounts by Google’s growing ambitions in advertising online, in print and in broadcast media.