When it comes to becoming a force in social networks, Google and Yahoo have tried and largely failed. To be sure, Google has Orkut, which is popular in Brazil and the Philippines, but not the United States. For its part, Yahoo has largely pulled the plug on Yahoo 360. But it is clear that MySpace and Facebook (and Bebo in the United Kingdom) remain firmly on top of the social network heap.

So now Google and Yahoo are taking another tack — turning themselves into social sites without building a social network.

“We are not trying to be another social network,” said Yahoo president Susan Decker on Tuesday, during the company’s earnings conference call. “Rather, by linking users’ favorite destinations and content, with their friends’ families and communities, we can deliver better relevance on a scale that no one else has achieved.” Two days later, the company’s new chief technology officer, Ari Balogh, speaking at the Web 2.0 Expo in San Francisco, elaborated a bit on the idea. “We don’t think of social as a destination,” Mr. Balogh said. “We think of social as a dimension.”

What does that mean? It means that Yahoo will attempt to bring the kinds of features that are common on Facebook and MySpace to its own sites. Users may be able to share photos, videos, news reading habits or calendars with their friends and receive alerts about what friends are up to. Who is a friend might depend on context. For instance, a user’s activities on Yahoo Sports may only be shared with that user’s fantasy sports playmates.

Google has long hinted that it would take a similar approach. Earlier this week, it suggested that users of iGoogle, a personalized home page service, might be able to share activities with friends. And the company has allowed users of Reader, its blog viewing service, to share items with friends.

One challenge both companies face, however, is how to turn the voluminous amounts of data about relationships that they have in their e-mail, calendar and other services, into “social graph,” a set of relationships establishing who is friends with who. They will have to tread carefully. Google’s efforts to determine who Reader users wanted to share items with was greeted with some alarm. At a lunch with reporters, Mr. Balogh promised that Yahoo wouldn’t force relationships on anyone without their consent.