CARLSBAD, Calif. -- Twitter's co-founders say the rapidly growing online communications company will eventually charge fees for its services, but it's unclear which ones and what will drive revenue. "There will be a moment when you can fill out a form or something and give us money," said Evan Williams, co-founder and CEO. "We're working on it right now," Williams said at the Wall Street Journal's D: All Things Digital conference. Williams and Twitter co-founder Biz Stone mentioned possible revenue-generators, including a service that would authenticate the source of information. For example, Dunkin' Donuts could pay to make sure that impostors don't send messages under its name. Still, after nearly one hour of questions from journalists Walt Mossberg and Kara Swisher and from the audience, the co-founders gave no clear picture of Twitter's business model. Stone demurred when asked what would be the company's key revenue driver in two years. Williams said he wasn't opposed to banner advertising but was unenthusiastic. "I think it's probably the least interesting thing we could do," he said. Williams said one of his top priorities was hiring more people to help grow the company but he didn't give a headcount target. San Francisco-based Twitter has 43 employees, he said, double its count in January. Twitter allows anyone to write about what they're doing or what's on their mind in messages sent through the Web or cellphones, also known as "tweets," which are limited to 140 characters. The unconventional, free service has attracted millions of users.