Facebook has switched on its Safety Check feature for users in the US following the attack in Orlando early this morning that left at least 50 people dead — the worst mass shooting in US history. The feature allows you to notify friends whether you're in the affected area and mark yourself and others as safe. Facebook's sidebar about the crisis describes it as a terrorist attack.
Facebook launched Safety Check in October 2014 after first introducing the feature in Japan due to the country's propensity for natural disasters. The social network's deployment of Safety Check has not been without controversy; Facebook had to explain why it was switched on for last year's Paris attacks but not other contemporaneous incidents, and earlier this year accidentally sent out supposedly local alerts to people around the world after a bombing in Pakistan.