Social media is a work of genius. It's a truly compulsive platform where people create content for free, generating traffic and interaction in a continual loop, growing bigger and bigger. Everyone and their newborn baby is now all over Facebook, Twitter, LinkedIn, Google+, Pinterest, Instagram, etc. etc. Social media is how people communicate and how they get their information. It is where people are daily. It is how they interact and socialise. It is how they support and protest. And I don't know if you've looked up from your phone lately but if not then just take a look right now, because in case you haven't noticed, it's like the Walking Dead out there. Seas of people plodding along; their eyes transfixed to the eerie glow of the smart device that might as well be surgically grafted to their grasping hand. And what is it they're all looking at? Facebook, Twitter, LinkedIn, etc. And what do they do when they see something they like, or something that makes them laugh, or something that inspires them, or something that outrages them?
They share it... on their mobile… to other people who are glued to their mobiles. And what do you suppose might happen if that content that they are sharing isn't mobile friendly? (It probably won't be getting shared in the first place.) But even if it does, when others are then exposed to it down the line they simply won't stick around and try to wrestle with that content if it doesn't just simply work on their mobile device, because that is where we've all come to expect. Let's refer back to those statistics above... 51% of time online in 2015 was spent on mobiles - that means in 2015, we finally crossed the Rubicon and now we have to start caring about experiences on mobile devices arguably even more than we do about desktop devices. And that number will keep growing.
So, responsive website design is essential (especially if you're hoping to be the next viral sensation and have billions of excited users flooding to your site from their mobile phones).