I advertised my free 2 days on here, Kindle Users Forum and KindleBoards as well as on Twitter, my Facebook page and Reddit.
As for reddit,
http://www.reddit.com/r/FreeEBOOKS/ is the most visited one and I got a lot of clicks and upvotes on there, but
http://www.reddit.com/r/KindleFreebies/ is also pretty well populated. Do read the reddit rules though and follow them when you submit your link.
As for my 2-day free offer I had 189 downloads on day 1, 54 on day 2 and, like John, 3 on day 3 (maybe a time-zone thing?). Got to #1 on the Top 100 Free Short Stories and gained one review so far from someone who followed me on Twitter after spotting my offer tweets.
As for twitter, even though I hate them, using the correct hashtags does increase the number of people who see your tweet so make sure you get #free and #amazon in there
Hope this helps,
Tony.