Online retailers and publishers are pushing back against Facebook Inc. 's efforts to track users across the Internet, fearing that the data it vacuums up to target ads will give the social network too much of an edge.

Web traffic experts say there is less data flowing from some sites to Facebook, suggesting they have been reprogrammed to hold back information.

Facebook has long kept track of the websites its users visit when they aren't on the social network. Three months ago, it began using the data to build more detailed user profiles, allowing advertisers to target people with more personalized marketing pitches.

That has rankled some retailers, advertisers and Internet publishers, which worry that the wider use of browsing history will hand Facebook, and potentially their own rivals, more information about existing and prospective customers.

In response, some businesses appear to have changed their sites to send less data to Facebook; others say they are considering such moves.