The Federal Trade Commission is giving a bit of pre-marriage advice to Facebook and one of its many betrothed, messaging app WhatsApp, which said “I do” to Facebook’s $19 billion (with a “b”) proposal back in February. Given Facebook’s past transgressions, the FTC felt that maybe it was worth reminding the giddy-in-love couple that there are laws about what they can and can’t do with users’ data.

In a letter [140410facebookwhatappltr] to Facebook’s Chief Privacy Officer and WhatsApp’s general counsel, the director of the FTC’s Bureau of Consumer Protection sits the two parties down for a serious chat about the temptations they might face when they merge their lives together.

“[B]oth companies collect data from consumers but make different promises and statements with respect to consumers’ privacy,” reads the letter, which points out that the protections offered by WhatsApp’s privacy policy “exceed the protections currently promised to Facebook users.”

Just like that uncle who tells you how marriage can change you, but shouldn’t, the FTC advises that “WhatsApp must continue to honor these [existing] promises to consumers.”

But while that stern uncle’s only recourse is a frown or a scolding if you let your spouse’s bad habits influence you, the FTC has the law on its side, reminding WhatsApp that a if there’s a failure “to honor these promises, both companies could be in violation of Section 5 of the FTC Act” and possibly in violation of Facebook’s 2011 settlement with the FTC over allegations that it failed to live up to its privacy promises.

Amid challenges from privacy advocates concerned that the Facebook acquisition could result in the gutting of WhatsApp’s current policies, the company’s CEO stated in March that “our principles will not change,” and that “We will never change our privacy policy.”

Facebook Chief Adolescent Billionaire Mark Zuckerberg echoed this claim, stating that “We are absolutely not going to change plans around WhatsApp and the way it uses user data.”

According to the FTC, these statements from all involved parties “constitute clear promises to consumers about the collection and use of their data” by WhatsApp and, if the merger goes through, Facebook.

A failure to keep such promises could be a violation of the FTC Act, and the letter reminds both companies that the commission has not hesitated to pursue actions against companies for such violations.

If the companies do eventually make changes to the WhatsApp privacy policy, the only way they could use data collected before that change would be if they receive affirmative express consent from the users. So even if WhatsApp does start to act like Facebook, it would legally be required to get users’ permission before any old data could be used in a new manner.

Another issue mentioned in the letter is that 2011 settlement between the FTC and Facebook, which enjoins Facebook and its subsidiaries from misrepresenting the extent to which they maintain the privacy or security of consumers’ personal information. The company is also required to obtain consent before sharing users’ non-public information in a manner that “materially exceeds any privacy setting.”

Should the companies ever decide to make changes to WhatApp policy, the FTC recommends that Facebook “offer consumers an opportunity to opt out of such changes or, at least, that you make clear to consumers that they have an opportunity to stop using the WhatsApp service.”

“Hundreds of millions of users have entrusted their personal information to WhatsApp,” concludes the letter. “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.”

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