WASHINGTON -- Sen. Al Franken called the ride-sharing company Lyft's response to his privacy questions "promising" Monday after the company delivered an eight-page letter vowing they were trying to be honest with customers about how they use personal data.

Lyft's letter comes a couple months after Franken pressed the San Francisco-based company, whose drivers cruise around in cars adorned with pink mustaches, to clarify how it uses customer data, including whether the company shares consumer information with third parties and how long the company maintains riders' personal travel information.

Ride-sharing companies like Lyft, and its larger competitor Uber, work by users "requesting" rides from roaming drivers using cell phones. The drivers find their riders by cell phone GPS tracking devices. 

"I appreciate the company's effort to provide thorough answers," Franken said, in a statement on Monday. "I look forward to further discussing these matters."

The second-term Democrat pressed both Lyft and Uber at the end of last year to be more transparent with how they bank and use consumer data after some unflattering stories broke about Uber threatening to publicize personal travel data about reporters writing critical stories about them.

Though Uber responded to Franken in December, the senator was deeply disappointed in that response, and said  he wanted more information from the company in the future. Franken said Uber's response fell short in how he expects the company to handle consumers' private travel information -- including when and where customers take cars and how long the company hangs on to personal rider information.

Franken is particularly sensitive about how companies use GPS tracking data and introduced a law last year, which didn't pass in the last Congress and would have to be re-introduced this year, that would give consumers more control over how tracking data is accessed on cell phones.

What comes next is unclear, though Franken said Monday he wanted to continue the conversation in this Congress.