People come up with all kinds of names for common phenomena  like the "Digg Effect" - if you get a spike of traffic from Digg you'll get some of that traffic, maybe 10%, as regular visitors who'll subscribe to your RSS feed, etc.

I had a spike in late May, got around 4000 visitors in 2 hours from a post I did comparing the Digg to the NY Times in terms of pageviews.   Not one comment came out of it - and FeedBurner did not really have a way to tell me if I picked up any subscribers because of a traffic spike like that.

The chart above comes from Darren Rowse and he mentions an uptick of RSS Subscribers that happens every time there's a Digg Spike. 

"…Every time I get dugg I notice a leap in my RSS subscriber count. The first few times a site gets on Digg this is often bigger than subsequent diggings (after a few times I guess those who are interested in your topic will have already been subscribed) but there is a noticeable bump in subscribers each time."

I don't have access to the FeedBurner account for this blog or else I'd try to tell if this is true.  It sounds like this is what should be happening - if I got 4000 visitors in 2 hours - it's reasonable that at least 1% (40) will become subscribers.  But I can't prove it-that this happened.

Maybe FeedBurner's newer metrics will allow more segmentation - I've been given access to the beta but it requires putting additional JavaScript on sites (which, in most cases, I can't do since I don't control the web template for Webmetricsguru.com, Artnewyorkcity.com or Smartmobs.com - in fact the only place I can do it is now-seo.blogspot.com - but I don't post there very often).