Internet.org, the Facebook-backed service that wants to bring free Internet to people in emerging markets, seemingly can't catch a break.

More than 60 civic and digital liberty groups from 28 countries signed an open letter addressed to Mark Zuckerberg on Monday contending that Internet.org in its present state violates net neutrality, thereby "threatening freedom of expression, equality of opportunity, security, privacy and innovation."

Internet.org, which Zuckerberg launched in 2013, partners with local telecomm companies and developers to offer a free, basic Internet experience with access to things like Facebook, Wikipedia and BBC News. Facebook disclosed earlier this year that Internet.org has helped more than 7 million people across six countries so far — Columbia, India, Zambia, Tanzania, Kenya and Ghana — and wants to be in 100 countries by the end of this year.

Zuckerberg views this as an "Internet for all"-type approach, but critics see things differently. Last month, for example, a group of Internet companies withdrew their support from Internet.org in India following a national backlash over net neutrality.

Advocacy groups this week took a similar tack, arguing Internet.org actually follows a "Zero rating" model, wherein service providers offer customers a set of services or applications that are free to use without a data plan or that don't factor in data plan usage. Worse still, they maintain, Internet.org could create a "two-tiered Internet," a scenario where new Internet users "get stuck on a separate and unequal path to Internet connectivity, which will serve to widen — not narrow — the digital divide."

Facebook, for its part, has tried addressing the criticism recently by rolling out the Internet.org platform earlier this month, which lets outside developers create complimentary services. In doing so, Internet.org is trying to chip away at the argument that it prioritizes some partnered content over the rest — a net neutrality no no.

“We and our critics share a common vision of helping more people gain access to the broadest possible range of experiences and services on the internet," an Internet.org spokesperson told Mashable in a statement.

"We are convinced that as more and more people gain access to the Internet, they will see the benefits and want to use even more services. We believe this so strongly that we have worked with operators to offer basic services to people at no charge, convinced that new users will quickly want to move beyond basic services and pay for more diverse, valuable services.”