aggregation tool

Can any statistician explain the clockwork regularity of many of these  
sites' popularity?
http://traffic.alexa.com/ 
graph?u=apple.com&u=dell.com&u=ibm.com&u=hp.com&u=sony.com
Most appear to have a sinusoidal curve with a period of a month.
Sony's is a poor fit, and Apple's maybe stochastic.

Alexa seems to be unavailable, so i am posting the most recent results.
Perhaps they are related.....

thanks

Jonathan Chetwynd
http://www.peepo.co.uk
"A web by people with learning difficulties"