According to the article, it apparently comes from advertising and subscriptions:
> Pavel Durov told the Financial Times that Dubai-based Telegram had grown to become one of the world’s most popular social media apps while making “hundreds of millions of dollars” in revenues after introducing advertising and premium subscription services two years ago.
> Pavel Durov told the Financial Times that Dubai-based Telegram had grown to become one of the world’s most popular social media apps while making “hundreds of millions of dollars” in revenues after introducing advertising and premium subscription services two years ago.