Most of the FTC staff has been furloughed due to the government shutdown, although the agency does have some employees still handling consumer protection issues.
A longtime privacy advocate during his years in the House, Markey now serves on the Senate Commerce, Science and Transportation Committee, which handles consumer privacy issues.
Google declined to comment on Markey's letter.
The company announced the planned change to its terms of service in a blog post on Friday.
If a user follows a local bakery on Google+ or rates an album as four stars on Google Play, that information could appear as an endorsement to their friends and family, Google explained.
The ads could appear on Google's websites or the millions of other pages that are part of Google's advertising network.
The company said it will not include information of people under 18 years old.
"We want to give you – and your friends and connections – the most useful information," Google said in the post. "Recommendations from people you know can really help."
The new policy is set to go into effect on Nov. 11. Users can block their information from being used in ads by adjusting the settings in Google's "shared endorsements" page.
The move follows in the footsteps of Facebook, which already includes people's personal information in "sponsored story" ads to their friends. Facebook's policy resulted in a class action lawsuit, which claimed that the social network changed its privacy practices without properly notifying users.
Due to previous privacy violations, both Google and Facebook have entered into legally binding agreements with the FTC that restrict how they can handle users' personal information. Google agreed to its settlement after it publicly revealed users' personal information such as Gmail contacts as part of the launch of its now defunct social network, Google Buzz.
The settlements bar the companies from misrepresenting their privacy policies or changing their privacy practices without first obtaining user consent. Violations of the settlements can result in hefty fines.