Basing the reply rate on the number of followers that an account has is fairly nonsensical. One the one hand, you have bots and inactive accounts, and essentially read-only use cases, and on the other hand the tweet went out to more than those 400,000 followers when you take into account retweets.

For example, I tweeted about the completion of my Toy Story 365 set to my roughly 40 followers last year: http://twitter.com/#!/Alanfalcon/status/9318186977595392

I also @ mentioned some influencers who might care about that sort of thing (Lee Unkrich, Disney/Pixar, the Pixar Blog) and so my tweet got retweeted by Lee and Disney/Pixar, which means the number of followers to whom my message was broadcast was more like 1,000,000 than 40. That day I had 23,874 hits on my flickr page (see the screenshot on my blog here - http://www.syncingdreams.com/2010/12/reward-for-project-comp... ... note that those numbers are not 23,874 uniques–many people clicked through all 365 photos, many more clicked a handful of them). I also had a dozen re-re-tweets and @replies to myself on Twitter, and maybe 6 comments added on my flickr stream itself.

Would you say that my community of 40 (43 including the @mentioned folks) resulted in 6 decent quality replies? 10 replies (including both flickr comments and twitter comments)? Or that my potential audience of more than a million resulted in just 10 replies? How would you determine the potential audience size anyway? By adding all the followers of those who retweeted my link? Do you go through and remove all the duplicates who are following more than one re-tweeter?

Anyway, all of this to come to the conclusion that the numbers presented in this post are pretty much useless without a lot of context, and also to present my own useless numbers as a kind of counter-point.