On Black Friday, one of the busiest shopping days in the U.S., retail stores aren't the only ones that get hit with tons of traffic. According to Keynote Systems Inc., a research group that tracked the performance of around 30 online retailers, Sears.com went down for two hours with a message telling shoppers to try again in a few minutes. Sears.com was the only site to go down, though other sites experienced some minor slowdowns.