Some erosion in primetime viewing is due to increased usage of video games, according to Nielsen Games measuring division, says Media Daily News. 

Says the article, "Television viewer erosion is the result of a fractionalized viewership — which is due, in large part, to cable viewing and Internet usage. But a lot of video game usage is also contributing, especially in the evening hours between 7 and 11 p.m."

Specifially, the article says, "Xbox 360 usage, for example, hits nearly 25% between 7 and 11 p.m, with men around 23% and women’s usage at around 17%."