Entities that collect personal information for legal purposes include: Social networks that provide their services without user fees make a profit by selling advertising.
This means that advertisers and others are able to use information gleaned from social networks to build a profile of a user’s life, including linking browsing habits to one’s true identity.
Read Krishnamurth and Will's 2009 study On the Leakage of Personally Identifiable Information Via Online Social Neworks.
This practice is appealing to marketers because targeted advertisements are more likely to result in a purchase by a viewer than comparable non-targeted advertisements.
They are valuable to social networks as they can be sold at a higher price than regular ads.