Safety Check automatically sends users in an affected area a note asking if they are safe.
When a user clicks on "yes, let my friends know," the tool then notifies their Facebook friends.
In a statement, a Facebook spokesperson said: "We hope the people in the area find the tool a helpful way to let their friends and family know they are OK."
French Prime Minister Manuel Valls faced boos and calls to resign as he laid a wreath for the 84 people killed in the lorry attack in Nice.
Hours later Mohamed Lahouaiej Bouhlel drove a 19-tonne lorry down the promenade killing 84 people and injuring 200 more.
France's prime minister stated on Sunday he believed the man behind the Nice lorry attack was radicalised "very quickly"