Suprisingly OSM does not have a lot of users in USA for it's size. All data (incl. map data) from the US government is public domain, so the USA has access to a lot of free map data. But in Europe (specifically the UK), the government map data is heavily closed. Hence OSM was started in the UK.
But is that public domain data any good? Mappers in the US are still using MapRoulette to clean up Tiger imports etc.
I think it's also that many Europeans, especially Germans, are very sceptical of google while in the US google gets a lot of love, especially these days with fiber.
> But is that public domain data any good?
No not really. Some people think the import into OSM of TIGER data has hindered the creation of a US OSM community, because the map looks "good enough"/"looks done", when it in reality it's not.
There totally is a party in SF! http://www.meetup.com/Maptime-SF/events/199399492/
Weird, no party in SF or Austin? Finally, the "hypest" tech cities are the ones which are the less involved in OSM, that's a surprise!