Facebook has not yet activated its Safety Check feature after the Brussels attacks, despite repeated requests that it do so.
The tool allows users to mark themselves as safe if they are thought to be in the area of a tragedy or natural disaster.
But the site has been criticised for only enabling it in certain instances, and sometimes taking time to do so.
Explosions in the Brussels airport and Metro have killed 13 people and injured 35, according to reports. It is still unclear the cause of the explosions.
Many social media users are asking the site to turn on the feature so that they can find out if people they know are safe or so that they can mark themselves as being. The problems with communication have been made worse by the phone networks apparently getting overloaded and breaking down.
Facebook turned on the Safety Check feature for an attack for the first time after the Paris shootings late last year. Before that, it had only been used for natural disasters.
Since then it has been repeatedly criticised for only turning on the feature selectively, and avoiding some attacks, especially in the developing world or not in the West.
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