Perhaps a more interesting — and more accurate — way to figure out where college students are going online is to assess which of the 172 web categories tracked by Hitwise get the most hits from 18- to 24-year-olds. Here"s a shocker: Porn is not No. 1. I"ve actually been puzzled by the decrease in visits to the Adult Entertainment category over the last two years. Visits to porn sites have dropped from 16.9% of all site visits in the U.S. in October 2005 to 11.9% as of last week, a 33% decline. Currently, for web users over the age of 25, Adult Entertainment still ranks high in popularity, coming in second, after search engines. Not so for 18- to 24-year-olds, for whom social networks rank first, followed by search engines, then web-based e-mail — with porn sites lagging behind in fourth.