There's biking of all kinds here and all types of cultures. Steel bikes are typically seen more around the east side/downtown, though it is hardly a rule. Orange 20 bikes and the Bicycle Kitchen are the local hubs for those folks. They're typically casual/urban riders and younger.
The more serious (and wealthier) people tend to go carbon for performance and/or bling. You'll see those in the mountains and on the faster group rides.
Many people have both steel and carbon.
There's an LA Bike group on ****** with some good info.