Twitter was first launched as a public service way back in 2006 and has seen its user base grow into the hundreds of millions of people in the years since. With its meteoric growth, Twitter has risen into the pantheon of modern social media sites along with Facebook and LinkedIn.

One of the reasons Twitter is so popular is how easy the site makes it to share ideas and information. Activists and opinion makers of all persuasions have adopted Twitter to reach out to new audiences and to share news and thoughts with existing followers. The environmental movement, never one for lagging behind in terms of marketing technology, has jumped into the Twittersphere with full force. Green tweeters offer a constant flow of quality tweets highlighting important stories that their followers should be aware of. They help make connections between people doing good work for the environment and they call out the bad guys for closer scrutiny and action.

We spent some time sifting around the environmental Twittersphere and put together this list of 10 people who are working to save the world who you should follow on Twitter. Follow them, retweet them, and share their accounts with your friends and followers. The world needs to hear more of their messages.

@gfriend, aka Gil Friend

Who he is: Gil is the founder and CEO of Natural Logic, a well-respected sustainable business and management consulting firm. He's a published author, a prolific blogger and speaker, and has a black belt in Aikido.

Why we follow him: Gil tweets well and he tweets often. On a busy day you might get more than 30 tweets from Gil, each of them a good news story, thoughtful retweet, or interesting conversation with other green players. Follow Gil and you will absolutely be in the loop of things, environmentally speaking.

@StarFocus, aka Danielle Brigida

Who she is: Danielle Brigida is the top social media strategist at the National Wildlife Federation and a well-respected leader in the field of nonprofit social marketing. She almost single-handedly brought the NWF into the social media age and built their profiles from zero to more than 145,000 Facebook fans and 158,000 Twitter followers. She's highly sought after as both a speaker and interview subject.

Why we follow her: There's a reason why Danielle is such a well-respected social media maven — she's a master of the medium. Her tweets are informative, catchy, fun and smart. She engages with her followers and does it all with a wonderfully human voice.

@BillMcKibben, aka Bill McKibben

Who he is: Bill McKibben is a prominent writer, activist, educator and environmentalist who founded the climate change campaign 350.org and who was called "the world's best green journalist" by Time magazine. He's recently taken up the cause of stopping the Keystone XL oil pipeline project and spent three days in jail in 2011 after he was arrested while protesting the plan.

Why we follow him: Bill's tweets do a great job of pulling together all the disparate ties that blend to tell the story of climate change.

@AdamWerbach, aka Adam Werbach

Who he is: Adam first burst into prominence as an environmental activist when he was elected president of the Sierra Club in 1996 at the age of 23, the youngest person to hold the seat. Since then he has widely spoken and written books about environmental activism and today runs sustainability for advertising firm Saatchi & Saatchi, oversees the launch of his startup Yerdle, a social platform for giving unwanted things away to your friends (and vice versa), and covers the environment for The Atlantic.

Why we follow him: Adam does a good job of leveraging his extensive social networks into interesting Twitter conversations while sharing interesting news and retweets.

@Kate_Sheppard, aka Kate Sheppard

Who she is: Kate works as a senior reporter and energy and environment editor at the Huffington Post, having previously written for both Grist and Mother Jones. She keeps a sharp eye on the intersection of politics and the environment and has been an important voice in covering subjects like fracking, the XL Keystone pipeline project, and climate change legislation.

Why we follow her: Kate's Twitter feed is the feed of a passionate and dogged journalist, popping with timely news links and interesting questions. She does a lot of retweeting and shows a deft touch in curating what she shares.

@AlGore, aka Al Gore

Who he is: He's Al Gore. You know who he is.

Why we follow him: Al's feed is a good source for news about the global battle against climate change apathy. He's an active Tweeter and intersperses personal notes about his life within his news links and the rare (and highly coveted) retweet.

@TheSoapGroup, aka John Rooks and Brek Heikkinen of The Soap GROUP

Who they are: John Rooks and Brek Heikkinen lead the charge at The SOAP Group, a small sustainability, marketing, and advertising agency located in Portland, Maine. What the SOAP Group lacks in size, they make up for in positive environmental impact, helping companies and organizations like Interface Carpet, Clif Bar, the World Bank and the National Parks Service do well by doing good. They are rabid opponents of greenwashing and developed the marketing principle More Than Promote, which holds that companies should use their marketing dollars and energy to not only tell their stories, but to also make a positive impact on the world at large.

Why we follow them: Actively following the SOAP Group's Twitter feed is like being right in the trenches in the battle against corporate greenwashing. John and Brek retweet important thinkers and ask thought-provoking questions about what it really means for corporations to act sustainably.

@TheGoodHuman, aka David Quilty

Who he is: David Quilty is a longtime environmental writer and publisher best known for his website The Good Human, one of the oldest blogs in the green blogosphere. David has a been a constant and passionate voice in the world of environmental health and his writing has been read by millions of readers.

Why we follow him: David does not pull any punches in his Twitter feed and isn't afraid to drop a healthy dose of snark into the mix. He tinges his Twitter voice with just the right amount of outrage and leaves you a more informed follower.

@sroakes, aka Summer Rayne Oakes

Who she is: Summer is the rare combination of a fashion model, environmental scientist, and entomologist (she's a "bug scientist"). After graduating from Cornell University, she worked as a television correspondent for Discovery's Planet Green, started her own fashion consulting firm, wrote a well-received book, and is a frequent speaker on how the fashion industry should embrace sustainability.

Why we follow her: Summer is a frequent tweeter and offers a robust stream of links about health, science, climate change and current events. Her voice is as enthusiastic as it is smart and is not one to miss.

@MarcGunther, aka Marc Gunther

Who he is: Marc Gunther is a veteran journalist who covers business and corporate sustainability for Fortune magazine. He's written for sites like Slate, GreenBiz.com, Wired, and the Guardian and maintains a popular blog at MarcGunther.com. (Though we share the same last name, Marc and I are not related).

Why we follow him: Marc offers a much needed voice in the worlds of business and sustainability. He has no doubt that human activity is causing climate change but thinks that markets might have better solutions than government legislation. He thinks that Walmart can be good for the environment but has no problem pointing out how much DuPont and DowChemical pollute.