It's a fascinating but frightening article.
http://online.wsj.com/article/SB10001424052748703940904575395073512989404.html?mod=WSJ_hpp_LEFTTopStories
The Journal found that Microsoft Corp.'s popular Web portal, MSN.com, planted a tracking file packed with data: It had a prediction of a surfer's age, ZIP Code and gender, plus a code containing estimates of income, marital status, presence of children and home ownership, according to the tracking company that created the file, Targus Information Corp.
One such ecosystem is Inc.'s ad network, which collects fees by placing targeted advertisements on websites. 's network knows many things about recent high-school graduate Cate Reid. One is that she is a 13- to 18-year-old female interested in weight loss. Ms. Reid was able to determine this when a reporter showed her a little-known feature on 's website, the Ad Interest Manager, that displays some of the information had collected about her.
