Apple and Google are headed back to Washington—D.C., that is—for another hearing on mobile privacy. The two companies will be joined by Facebook this time in order to testify in front of the US Senate Subcommittee on Consumer Protection, Product Safety, and Insurance on "Consumer Privacy and Protection in the Mobile Marketplace." The hearing will take place on Thursday, May 19 at 10am Eastern Time.

Apple will be sending Catherine A. Novelli, Vice President of Worldwide Government Affairs, to testify on its behalf on Thursday, while Google will once again send Alan Davidson, Director of Public Policy for the Americas. Facebook's Chief Technology Officer Bret Taylor will be joining the group as well, along with David Vladeck from the Federal Trade Commission, Morgan Reed from the Association for Competitive Technology, and COO of Common Sense Media Amy Guggenheim Shenkan. Senator John D. Rockefeller (D-WV) will chair the hearing.

This is a largely different group than the one that testified last week in front of the Senate Judiciary Subcommittee on Privacy, Technology and the Law. At that time, Apple VP of Software Technology Guy "Bud" Tribble and Google's Davidson faced some hard questions from the FTC, US Justice Department, and several privacy groups about mobile device tracking, location-based services, DUI checkpoint apps, and more.

Senator Al Franken (D-MN) was not particularly impressed by many of the answers he got out of Apple about location tracking and privacy, however, so it won't be a surprise to hear some of those same questions pop up during this week's hearing. Apple general counsel Bruce Sewell told US Representative Ed Markey (D-MA) in a letter last week that the company did, in fact, share some of its anonymized location information with a partner—a detail that will undoubtedly come up when Apple faces the panel on Thursday.

As for Facebook, it's likely that the company will face questions related to its most recent privacy-related incident wherein security firm Symantec discovered that "hundreds of thousands of applications" might have leaked personal information about users to third parties. Though Facebook allegedly investigated the matter and claims that there's no evidence of actual data leakage, observers (including, once again, Ed Markey) want more information about what happened and why.

At first blush. Facebook seems like a wildcard in the company of Google and Apple on this topic—after all, Facebook doesn't have a mobile device OS that tracks user locations (yet). However, Facebook has apps on nearly every platform, and has recently joined the check-in and local deals game itself, so the company's position on privacy is even more important as users begin attaching their location to Facebook postings.

Listing image by Python Monty Pictures