I think there’s another effect of such big traffic spikes – once it happens, it gives you an opportunity, using the next few posts you write, to retain some of that Digg audience.

I say this not as a blogger but simply as a reader. I will often add a blog to my RSS collection or my bookmarks after following a Digg/Reddit/whatever link. If the next few posts are good, I’m likely to keep it. If they’re not, it’s gone.

It puts a little extra pressure on the blogger, but even if only 1% of those 18,000 visitors became regular readers, that’d still be 180 new regular readers.

That said, I have no idea if there is any research on Digg Effect retention rates and what helps/hinder it.