As a host of countries move to rein in social media use by children, could this be technology’s big tobacco moment? Tiago Rogero in Rio de Janeiro Sam Jones in Madrid   Angelique Chrisafis in Paris   Natasha May in Bangkok Helena Smith in Athens   Leyland Cecco in Toronto   Hannah Ellis-Petersen in Delhi Jennifer Rankin in Brussels   Dan Milmo in London Kate Connolly in Berlin   Miranda Bryant    Alastair McCready in Taipei Rachel Savage in Johannesburg   Ankita Rao Nick Robins-Early