His comments, sitting at a circular table with EU Parliament leaders, dressed in a suit, tie and white shirt, echo an apology last month to U.S. lawmakers.
But questions remain over how Facebook let the leak happen and whether it is doing enough to prevent a recurrence.
Zuckerberg’s appearance in Brussels comes three days before tough new EU rules on data protection take effect. Companies will be subject to fines of up to 4 per cent of global turnover for breaching them.
Zuckerberg stressed Facebook’s commitment to Europe, where it will employ 10,000 people by the end of the year, he said. “I believe deeply in what we're doing. And when we address these challenges, I know we'll look back and view helping people connect and giving more people a voice as a positive force here in Europe and around the world,” he said.