As Facebook pushes forward with its acquisition of popular messaging service WhatsApp, the Federal Trade Commission today released a letter clearing the sale, but warning both companies not to forget about their obligations to consumer privacy. Jessica Rich, director of the FTC's Bureau of Consumer Protection, said that WhatsApp must hold true to all privacy-related promises the company made to users after the buyout was first announced. "WhatsApp has made a number of promises about the limited nature of the data it collects, maintains, and shares with third parties — promises that exceed the protections currently promised to Facebook users," reads the letter. "We want to make clear that, regardless of the acquisition, WhatsApp must continue to honor these promises to consumers."

If WhatsApp fails to meet its end of the bargain after being scooped up by Facebook, both companies risk violating FTC regulations and a separate settlement Facebook reached with the commission in 2011. "Before changing WhatsApp's privacy practices in connection with, or following, any acquisition, you must take steps to ensure that you are not in violation of the law or the FTC's order," the letter says.

The FTC then lays out three guidelines that will be key to avoiding trouble. First, if WhatsApp eventually starts using collected data "in a manner that is materially inconsistent with the promises WhatsApp made at the time of collection," it must obtain affirmative consent before doing so. The company is also forbidden from misrepresenting the extent to which it protects WhatsApp user data. And finally, if WhatsApp suddenly changes how it collects, uses, or shares new data, the FTC is urging the company to let users opt out — or at the very least "make clear to consumers that they have an opportunity to stop using the WhatsApp service."

Today's letter doesn't raise any other objections to the acquisition, but the commission remains keenly aware of Facebook's past controversies. "Hundreds of millions of users have entrusted their personal information to WhatsApp," reads the letter's conclusion. "The FTC staff will continue to monitor the companies' practices to ensure that Facebook and WhatsApp honor the promises they have made to those users."

Update: Apparently the FTC has given the all-clear for Facebook's acquisition of WhatsApp, with a spokesperson telling Mashable "We're pleased the FTC has completed its review and cleared our acquisition of WhatsApp."