Online behavioral tracking has increased significantly in the last two years, according to a new study by Krux Digital, an online data analysis firm based in San Francisco.

The Wall Street Journal reported the increase on Sunday in advance of the study set to be released on Tuesday. Its findings? The “average visit to a Web page triggered 56 instances of data collection, up from 10 instances when Krux conducted its initial study, in November 2010.” That's an increase of more than 400 percent in less than two years.

The company says it automatically crawled several pages on each of the 50 most-visited sites as measured by comScore, an online traffic monitoring firm. That would include the main properties of Google, Microsoft, Facebook, Amazon, Wikimedia, Apple, Yelp, Netflix, and others. Krux did not say which pages it specifically looked at.

Such behavioral tracking is largely fueled by the rise of “real-time bidding exchanges,” where data gathered from users is sold at lightning speed to the highest bidder as a way to sell increasingly targeted ads to Web users.

Just last week, Facebook said it would be using this technique for advertisers “within weeks.”

WSJ also reported that Forrester Research, an analysis firm, “estimates that real-time bidding will constitute 18 percent of the online display-ad market this year, up from 13 percent last year.”

"It's gone from virtually zero in 2009 to about a fifth of the entire market right now," Michael Greene, a Forrester senior analyst, told WSJ. "We've moved from a traditional advertising model of buying 1,000 impressions. Now you evaluate and buy a single impression."

However, some analysts view this type of tracking to be a net benefit.

“While ads targeted in real time can seem big-brotherish, it's important to understand that while these services have access to your browsing history, which is in a sense personal information, they don't have access to personally identifiable information,” wrote Rebecca Lieb, an analyst with the Altimeter Group, in an e-mail to Ars on Monday. “They don't know who you are, or where you live, for example.”