874,000,000 people use Facebook on mobile devices every month. Now they should watch out that its new News Feed video element doesn’t push them over mobile data allowance. The new feature will show video adverts and videos published by your friends in the content that you’re shown.

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But videos posted by friends might take up some of users’ data allowance if watched when not connected to Wi-Fi. Although the ads will not be downloaded on mobile, it’s just "for now", according to Facebook. The company said that they will be downloaded while users are on Wi-Fi, but never specified how long this “for now” will last. The problem is that other videos that people have posted to News Feed will download wherever the users are unless they are blocked in settings.

Facebook announced that users can allow some of those videos to play automatically over Wi-Fi only, but it is unclear how many is “some”. So, in case you do not want your phone bill to rocket, you may want to disable video downloads over mobile data.

The company started introducing people’s video into News Feeds a few months ago, but didn’t clarify whether its downloads would happen over Wi-Fi or mobile. Only in December Facebook said that it was offering marketers a new way to tell stories in News Feed via video ads. The developers claim that on mobile devices, all videos that start playing as they appear on the screen will have been downloaded in advance if the device is connected to Wi-Fi. This means that such content won’t consume data plans, even if people are not connected to Wi-Fi at the time of playback. However, the wording of the announcement seems to imply it won’t always be the case.

It is also unclear what happens with those who do not use the mobile app, but view Facebook on the web – such users are recommended to check settings on phones too.