"I would definitely side with Tim Cook on this," he said on Tuesday, speaking at Mobile World Congress in Barcelona.

The surprise may be just how much his opinion matters. Less than three years after Durov and his brother launched Telegram, he now reports that 100 million people use the free encrypted messaging app every month, up from 60 million people last May. That growth is coming from all over the world. While that's a small number compared withthe billion people who pull up WhatsApp every month, or the 800 million people who go on Facebook Messenger, it's illustrative of the early growth that signaled each of these services had mainstream appeal. "Every day, 350,000 new users sign up for Telegram," Durov said. "And we have zero marking budget." In other words, Telegram is not going away.