Thanks. Valid feedback. Yes - we're missing that bit when a user joins with email address and without friends / followers. Will introduce that soon.
Personally, I hate spam as much as you do. We have enabled sharing on Facebook / Twitter only with a context. Almost every place with explicit user permission. There are many annoying apps, we don't want to get there.
Twitter should have been little more considerate in its communication with its developer community. Anil Dash made all that sound comforting/positive. A must leason for all platform enablers.