I recently attended a business meeting with a wide mix of businesses represented, and a presentation on social media. In the presenter's opinion, social media had made websites outdated.
If social media supplies a need for your business, that's great; we're making use of social media too, and many of our customers do, in different ways. But what it doesn't do is obviate the need for other forms of e-marketing and e-provision (not all websites exist for marketing!).
Take a business that doesn't sell anything online, doesn't have a great deal of information to get across, and doesn't have any online provision for such functions as customer/member service. Do they still need a website?
Assuming they sell something that people look for on the internet, then yes, I would suggest a website even if they have a social media presence on various platforms, simply to provide a base for describing what they sell, and pointing to the various social media sites.
The one constant in social media (if we are talking about facebook, twitter, pinterest.. - menshn closed last year, by the way) is that. as a user, you are faced with a rolling list of new topics, latest at the top.
So as a business, if you have to constantly re-state your USPs in order to keep them in people's minds, you're using the wrong medium!