Internet.org grants access to only a limited number of Internet destinations, including Facebook. In India, that has led a number of start-ups to pull their support.
In a blog post Friday, Zuckerberg said he strongly disagreed with characterizations that Internet.org violated the spirit of net neutrality. Universal connectivity and net neutrality, he said, "can and must coexist."
"To give more people access to the internet, it is useful to offer some service for free," he wrote. "If someone can't afford to pay for connectivity, it is always better to have some access than none at all."
He noted that Internet.org "doesn't block or throttle any other services or create fast lanes -- and it never will."
"We're open for all mobile operators and we're not stopping anyone from joining. We want as many internet providers to join so as many people as possible can be connected."
After a commenter asked Zuckerberg why Internet.org only gives access to select sites, the Facebook chief executive responded:
Internet.org offers free access in local languages to basic Internet services in areas including jobs, health and education. Already, more than 800 million people in nine countries can access those services through the initiative, Zuckerberg said.
"Arguments about net neutrality shouldn't be used to prevent the most disadvantaged people in society from gaining access or to deprive people of opportunity," he said.