Facebook's free mobile data scheme Internet.org has been criticised by digital rights groups from around the world, who say the initiative is unfair and threatens net neutrality.
In an open letter, 67 groups from countries such as Uganda, Ecuador and Indonesia have stated their concerns about the Internet.org app, which gives users access to a limited number of online services without incurring any data charges.
The scheme was created alongside partnering mobile carriers in parts of Africa, Asia and South America to bring some internet access to parts of the world where it was restricted - part of Internet.org's overall goal of getting the entire planet online.
Selected services, including Wikipedia, BBC News, Facebook and some local news providers would become available via the scheme's app without any data charge applying.
However, digital rights groups who signed the letter say the project threatens freedom of expression, privacy and the principle of net neutrality, which is the idea that all data is treated equally online, because only selected services can take advantage of it.
"It is our belief that Facebook is improperly defining net neutrality in public statements and building a walled garden in which the world's poorest people will only be able to access a limited set of insecure websites and services," said the letter.
"Further, we are deeply concerned that Internet.org has been misleadingly marketed as providing access to the full internet, when in fact it only provides access to a limited number of internet-connected services that are approved by Facebook and local ISPs.
In its present conception, Internet.org thereby violates the principles of net neutrality, threatening freedom of expression, equality of opportunity, security, privacy and innovation."
Facebook is yet to officially respond to the letter, but earlier this month the social network's co-founder Mark Zuckerberg defended the site's determination to get more people online.
"If a local fisherman gets access to free internet services he couldn't otherwise afford to help him sell more fish and support his family, then that's good, and we shouldn't have rules that prevent that," he said.
"Access equals opportunity. Net neutrality should not prevent access, we need both. It's not an equal internet if the majority of people cannot participate."
However the digital rights groups who contributed to the letter, which include the European-based Free Press Unlimited, says that Internet.org restricts choice, and is alienating local start-ups in the countries in which it is active, these include India, Tanzania, Ghana and the Philippines.
The letter also criticised Facebook for requesting that sites wishing to join the scheme not include encryption technology, which the campaigners said "inherently puts users at risk, because their web traffic will be vulnerable to malicious attacks and government eavesdropping".
However, Facebook has said that the Internet.org Android app will begin to support some encryption "in the coming weeks".
