Most of us will shoot off immediately that any company that wants to be successful does need a website that is separate from the social media platforms. It is a place for business to be ran. A place to allow the company to share details that a social media page may not be able to give.
Then again, the audience is attached at the hip to social media. Contact information for the company is easily found on an "about" section of the page. Cool videos and media is shared on the page for easy consumption. Social media pages are quickly identified as the "home" for many companies, rather than their actual websites.
It boils down to the fact of revenue.
Revenue comes in many forms when it comes to a presence online. Generally, it comes from sponsorships, and advertisements. Let us focus on ads right now. Social media pages don't give you the option to dig into any "ad revenue sharing" options. They don't exist on the popular platforms. There has been a couple social media platforms that have literally come and gone that offered a sense of "revenue sharing" for those that had content that went viral, but this presents a completely different problem we will discuss later.
A website allows a company to run advertisements. Services such as Google Adsense allows a website to host ads with little to no input from the user. The ads will generate based on what information that Google has mined from the visitor to promote content that is relevant to them. No market research on the company's end required. This is great for small companies.
Another issue you come across is getting the audience to actually go and view your content on your website. I wish I had a magic answer that would help everyone here, but I don't. Having interesting, and engaging content is important, but it doesn't always work. You have to have enough of an audience that is literally willing to leave the confines of the social media platform they are using. You also need to figure a method to create interesting ways to converse the topic. Using integrated Facebook comments is great to easily get your posts shared without the commenter even needing to click anything but the "submit" button. Disqus has a wonderfully integrated platform that does the same, but doesn't offer much in ways to tie back to social media platforms, and may scare away some potential commenters. Again, no secret answer here.
I have found it easier to directly post content that is small (about a paragraph or two) directly to social media makes more sense. I personally do not like to click a link to an article that leads me to the pages website that offers me a "snack" of information, rather than an actual "meal". As a member of the audience, I felt that it would have served me just fine on Facebook or Google+ with an attached image. It is the long pieces, like this article you are reading here, that makes more sense to me to be placed on the actual website.
All-in-all, it is important to have a presence on Social Media platforms, but also important to have an independent presence online via a website. You can get a little bit more personal with a website, and really demonstrate the uniqueness that you bring in comparison to the competition.