Unauthorized, but not illegal, “web scraping” of personal data is big business – $840 million according to an estimate by the Wall St. Journal.

The market for personal data about Internet users is booming, and in the vanguard is the practice of “scraping.” Firms offer to harvest online conversations and collect personal details from social-networking sites, résumé sites and online forums where people might discuss their lives.

The emerging business of web scraping provides some of the raw material for a rapidly expanding data economy. Marketers spent $7.8 billion on online and offline data in 2009, according to the New York management consulting firm Winterberry Group LLC. Spending on data from online sources is set to more than double, to $840 million in 2012 from $410 million in 2009.

The Wall Street Journal’s examination of scraping—a trade that involves personal information as well as many other types of data—is part of the newspaper’s investigation into the business of tracking people’s activities online and selling details about their behavior and personal interests.