"Our reading habits online encompass everything we're thinking about, political and religious views, health and relationship problems, " said Peter Eckersley, a senior technologist at the Electronic Frontier Foundation, a privacy-advocacy group.

WSJ: 'Like' Button Follows Web Users

According to Peter Bell, who has been a chief technology officer and a senior technologist at a number of startups including Skinnio and General Assembly, not all app developers are created equal.

WSJ: App Building, the Do-It-Yourself Way

"The product has been designed to make it easier to share with one group of your friends while retaining some measure of privacy with respect to your family, coworkers or other groups of friends, " said Peter Eckersley, a senior technologist at privacy-advocacy group Electronic Frontier Foundation, in an email.

WSJ: Google+ Pulls In 20 Million in Three Weeks

"Facebook needs to have a few very simple high-level controls" so users can keep data private, said Peter Eckersley, senior staff technologist at the Electronic Frontier Foundation.

WSJ: Facebook Grapples With Privacy Issues