“In retrospect, it might have been a mistake to give Facebook all of my personal information in exchange for seeing what my high school friends eat for dinner.”

So one Twitter wag quipped a couple of days after news broke that Cambridge Analytica, a consultancy firm, had harvested information from Facebook about tens of millions of Americans, and may have used it to try to influence the 2016 U.S. presidential election.

Facebook greets you on its login page with the slogan “It’s free and always will be,” but that’s not strictly true. Users pay to use Facebook by supplying their personal data, photos, videos, and much more. The pages, posts, pictures and videos that users “like” are tracked, and, with special software, can be used to predict the user’s buying habits, movie, book and music tastes, and even their political leanings.

That’s valuable data.

But as the Cambridge Analytica case shows, there may be other hidden dangers to providing Facebook all that valuable personal data for free. The company, according to reporting by the Guardian, New York Times, and the UK’s Channel Four News, used a vast data set harvested from Facebook users to perform psychological analyses of millions of U.S. voters. It then targeted them with political advertising designed to influence the way they voted.

There are a host of ethical issues connected to this case – not least the fact that much of the personal data was harvested without many users’ explicit permission or knowledge, and Cambridge Analytica may have illegally coordinated its actions with the campaign of then-candidate Donald Trump.

But the most important one is that the data users provided to Facebook may very well have been used to affect the outcome of a democratic election, and possibly the referendum in the UK on leaving the European Union. That’s not something Facebook users signed up for.

Facebook owes its users, and indeed the whole of democratic society, an explanation of how its data was allowed to be used in this dangerous way, and assurances that it won’t happen again. Our democracies are too valuable to be swapped for the chance to keep up with old friends’ dining habits.