Yahoo has embarked on an ad strategy based on that most elusive of online goals: building a "community" around a particular topic. It's planning 100 Web sites, each around one hot entertainment brand, fueled by its social networking and content tools.

Called Brand Universe, Yahoo launched its first site, around the Nintendo Wii video-game console, and by July it will launch ones centered on the Harry Potter books, video games Halo and The Sims, TV shows Lost and The Office, and Transformers toys. Yahoo's betting social networking and content aggregation can drive the sites, with little original content and no official brand tie-ins, and that advertisers will like the focus. "We can speak to a particular audience," says Sean Atkins, a Yahoo exec working on the effort.

It faces an uphill battle, though, in drawing site visitors. "It's a good idea from an advertising perspective," says Forrester analyst Charlene Li, "but how are you going to get visitors there?"