CANNES, France — Advertising agencies and Internet companies once viewed each other as foes, but are now coming together to harness the potential for online advertising. Like many other segments, online ad spending has slowed from its previous breakneck pace during the deep recession, forcing companies to devise new ways to chase fewer dollars.

Last week, Eric E. Schmidt, the chief executive of Google, and Steven A. Ballmer, his counterpart at Microsoft, for the first time attended an annual advertising industry meeting, the Cannes Lions International Advertising Festival...