The mammoth data centers that power the internet are greener than they used to be, but two major players – Amazon and Twitter – could do a lot better, according to the environmental watchdog Greenpeace.

By some estimates, Amazon is responsible for one percent of the internet's traffic. But Greenpeace says that only about 15 percent of that traffic is powered by green energy. In a report that the group is set to release on Wednesday, Amazon scored F's in a ranking of its commitment to renewable energy and where it locates its data centers. It also got an F on environmental advocacy, and a D in energy efficiency.

That's pretty much in line with the bad marks that Amazon scored back in 2011, when Greenpeace first started comparing the bit internet providers. Companies get better marks in Greenpeace's report if they work with their utility companies to seek out alternative power sources, such as wind power, and if they file annual environmental reports on their data center usage. Amazon hasn't done that. For example, Amazon's Oregon utility, Umatilla Electric Cooperative, is pushing the state to loosen environmental restrictions, says Greenpeace analyst Gary Cook. "If Amazon was motivated, you'd have a very different scenario unfolding in Oregon."

>The environmental group staged demonstrations at Amazon and Microsoft's Seattle offices – demonstrations that spurred Microsoft to go carbon neutral and sign long-term contracts for wind power.

Two years ago, the environmental group staged demonstrations at Amazon and Microsoft's Seattle offices, demonstrations that spurred Microsoft to go carbon neutral and sign long-term contracts for wind power. "We haven't seen a change in Amazon," says Cook. "Unfortunately, unlike many of those in the sector, they've not moved very much in the past few years." In an emailed statement, Twitter said it's trying to be more efficient. "Twitter believes strongly in energy efficiency and optimization of resources for minimal environmental impact," the company said.

Prior to its release, Amazon had no comment on the report. But after the report hit the web, the company emailed a statement saying that the report "misses the mark by using false assumptions" about Amazon Web Services, the cloud computing services that run inside the company's data centers. According to Amazon, it operates "efficient and highly utilized data centers" in 10 AWS regions across the globe, and two of those use 100 percent carbon-free power.

"We like offering customers the choice of being able to run carbon-free, and we love doing it without charging a premium over other North American regions, the company said. "Running IT infrastructure on the AWS Cloud is inherently more energy efficient than traditional computing that depends on small, inefficient, and over-provisioned data centers. With AWS, customers can reduce their overall consumption of IT resources while also improving utilization. Collectively, AWS customers are the driving force in this effort by eliminating hundreds of thousands of individual data centers worldwide, along with the associated wasted capacity and overprovisioned energy."

Facebook, Apple, and Google all scored failing grades in one category or another in Greenpeace's 2011 report. But not anymore. Since then, they've all made a serious effort to make sure that the energy that they use to run their data centers comes from renewable sources. For example, the three companies have pushed North Carolina's Duke Energy to give them ways to offset their dirty energy usage by purchasing clean power from outside the state.

Unlike Amazon and the others, Twitter doesn't actually build its own data centers. But Cook thinks the micro-blogging service could do a much better job when it comes to energy efficiency and disclosing its energy usage – both areas where it scored an F. "We have not seen any real evidence that they are managing their electricity supply chain," Cook says. But he hopes that by shining a light on the industry, Greenpeace might change that. "If we're going to make the internet a green platform, it can't just be the Googles and the Apples and the Facebooks."