I've definitely noticed that effect here on HN, and it doesn't seem so pronounced on many other sites. Here are the first sites I could think of, and they seem pretty flat. Sites like HN and TechCrunch have less content on weekends, so you'd expect a drop-off in traffic.
http://www.alexa.com/siteinfo/facebook.com
http://www.alexa.com/siteinfo/yahoo.com
http://www.alexa.com/siteinfo/msn.com