This seems pretty much in line with my own numbers. I had a post get to the front page for about ~36 hours and held one of the top few spots for a couple hours.

Over approximately 48 hours, I got ~35k uniques, approximately 18k were from Hacker News directly, with the bulk of the remainder coming from Twitter. Many of those twitter referrals came from people RTing HN aggregator feeds and the like though, so in reality probably more than that 18k number can be attributed to Hacker News.