Nine days ahead of the U.K.’s crucial referendum on the matter, with the markets freaking out about polls that suggest a lead for the pro-Brexit camp, Facebook suggested that users looking through “activities” suggestions in its status update tool may want to tell friends they were “in favor of leaving the EU.”

The option showed up for many English-speaking users of Facebook’s Android and iOS mobile apps, and not just in the U.K. The “in favor of remaining in the EU” option was there when searched for, but not shown by default—at least, for those who mentioned it on social media.

https://twitter.com/kukekomm/status/742699985465118720

.@jonworth has discovered Facebook's pro-Brexit bias https://t.co/og6vB5sr4b I can confirm for Android pic.twitter.com/Pd9iJLuDfn

— Valentin Kreilinger (@tineurope) June 14, 2016

However, while users expressed outrage at the apparent bias, a Facebook spokesperson said the phrase’s appearance would have only been based on other people’s activities.

“People who search for ‘EU’ when they are using our status tool will see a full range of options enabling them to add if they are thinking of leaving or remaining in the EU. People scrolling through the list of activities see options based on what their friends and other people have been using,” the spokesperson said.

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However, Facebook (FB) declined to give more details about how it weighs the activities or opinions of friends, and those of the wider population. For example, I saw the pro-Brexit suggestion on my mobile devices, but those in my circle of friends on Facebook tend to have the opposite opinion.

A few hours after people started raising questions, the pro-Brexit suggestion disappeared from the “More” menu, both in the Facebook apps on my devices, and those of others such as EU political blogger Jon Worth, who first flagged up the issue in a Tuesday post.

So at 1513 Facebook showed a pro-#Brexit bias. By 1854 they’ve removed it. No explanation. Transparency, eh? pic.twitter.com/I6qGo6pjnZ

— Jon Worth (@jonworth) June 14, 2016

As Worth noted, transparency is an issue here. Facebook has recently been accused of having a liberal bias to its “Trending Topics” section, and sociologists have long been warning about the ways in which Facebook can subtly influence wider political discourse.

For more on Brexit, watch our video.

It may well be that the pro-Brexit suggestion showed up in Facebook’s app because more users were expressing that sentiment. There’s certainly no reason to suspect that Facebook itself favors Brexit—like most multinationals operating in the U.K., it would probably rather see the country stay within the increasingly harmonized EU single market, for convenience’s sake if nothing else.

But even if the incident is a matter of the “wisdom of the crowds” becoming a nudge for individual users, it raises interesting questions about the political influence that the social network wields, intentionally or otherwise.