I have yet to meet a single person outside SF who uses Plancast more than once a month (from the five outside SF I know who use it at all). Not sure if I'd call it viral so much as quarantined.
Ok, two things :
1) i was looking at the viral/social features (graph integration, actions as "count me in", graph discovery, sharing features, ...
2) it's surprisingly taking off in europe, but with tech savvy early adopters, true.