It is very curious that I can go decades in San Francisco without running into any of the thousands of people I know and when I'm in Seattle, New York Tokyo or Los Angeles I always run into the few people I know.
The reason is clear. In San Francisco social networks are parallel and sorted by time of day and geography in a way that does not create overlap.
In the rest of America, societies are organized primarily by commerce which is a tree with trunk and branches that helps connect most people who are active in the commercial society. They are secondarily organized by social climbing.
People are striving for societal recognition of various sorts and as a consequence they make sure that they are in the locations and venues where they will be seen by other social climbers. This is particularly true in New York and the East Coast.
These two phenomenon, the commercial and the social climbing create overlapping social networks based on time and geography.
That explains this strange San Francisco phenomenon for me. If you need more let me know.