I haven't been to Half Dome in 25 years, and feel fortunate to have only seen a dozen other people on the trail when I went. Wow. Time to do what they do at Mt Whitney and have a lottery system for hiking permits.

there is a limited number of permits (http://www.nps.gov/yose/planyourvisit/hdpermits.htm) - a few friends and myself hiked whitney (got lucky, middle of the week, few applications) then drove to check out half dome but was not as lucky