Who would want to run a company nowadays? Wherever you turn your performance, attitude and culture are being challenged by consumers, the media, employees, NGOs and your own investors to be both successful and yet "responsible".

That pressure to be a "good" business is only going to increase. As the CEO of Body Shop, Jeremy Schwartz told the Guardian recently: "The idea of a company that made money but is also a force for good was laughed at 40 years ago. That's now mainstream."

No wonder then that four of the biggest (and overused) buzzwords in the world of modern corporate strategy are purpose, authenticity, trust and transparency. But how can you communicate your company's sense of purpose - what you stand for in simple terms - in a way the skeptics will believe?

Increasingly, the answer for many of the world's biggest companies is to highlight their sustainability work.

Why sustainability? Well, sustainability, after all, is rooted in doing business in a transparent and authentic way. And, in today's always-on, connected society, pledging to be a better company just won't cut it. Increasingly, executives are coming to terms with a Millennial generation of consumers that ask tough questions and do research on the ethical standards of the companies they buy from and aspire to work for. Business leaders know that this super-connected and informed populace poses serious challenges for their companies as they try to adapt, respond and stay relevant in real time.

The companies that are truly committed to integrating sustainability throughout their operations aren't doing this to look good - they understand the fundamental business value and opportunities it provides. They know that sustainable business planning won't just cut energy and natural resource costs, it will provide new business models along with product and service opportunities to meet the needs and expectations of a connected, changing society. Increasingly, sustainability will also give companies a crucial license to operate in countries, regions, states and cities where regulations enforce a more responsible way of working whatever their size.

That's why for the 6th Annual Social Media Sustainability Index we focused on what companies stand for and how effectively they communicate their sense of purpose. In 2015 GE and Unilever emerged as joint leaders of the Index. Both companies have multiple products, engagement strategies and awareness campaigns devoted to backing up their sustainability credentials and commitments (even though they don't necessarily use the term sustainability).

We studied the communication strategies of 475 companies in 2015 and selected a Top 100 for the Index. As you might imagine, different industry sectors look to play to their own strengths and specific sustainability concerns. Technology and media companies like Microsoft, Intel and Time Warner Cable stress the importance of science, technology, engineering and mathematics (STEM) education - an issue particularly close to home along with encouraging coding for kids.

Food and beverage companies now finally realize that people really do care what goes into the products they consume. So, in the last 12 months, the likes of General Mills, McDonald's and Unilever have looked to communicate their use of healthier ingredients and a closer relationship to farmers. Many of the biggest consumer brands, meanwhile, have taken a strong stand on social issues like female self-esteem, gender equality and LGBT rights.

Ultimately, whether you're a start up or a multinational good sustainability communications starts with having a compelling story to tell, and being able to back it up. After all, as in all form of business, if you can't walk the talk no-one will believe in you.