Yahoo grew as a place to stay. They built one service after another, hoping for time on site.
Google, on the other hand, began as a place to visit when you wanted to go somewhere else. That’s their entire business model. Time on site wasn’t as important to them as the accuracy of their direction. Come to leave.
Facebook, on the other hand, is organized to be a one-way street, with people staying on the site as long as possible.
Of course, it’s not simply web sites that work this way. Either we organize for junctions and trajectory, or we build our place as a destination, physically or as metaphor.