Buses are under-utilized, but walking and various forms of wheeled transportation (motorized longboards, those one wheel things, bikes, scooters, etc) are popular.

San Francisco is only about 7 miles wide. You can walk from one side of the city to the other in about 2 hours. When I lived there, I walked almost everywhere. The only times I did otherwise was when it was a really long walk (over 45 minutes), or I needed to be somewhere fast. The bus will not get you anywhere fast, it's only about twice as fast as walking. I think that's why a lot of people use Lyft. The buses are actually quite nice inside imo, they're just slow. Plus the stigma that only poor people ride the bus.