This drives me crazy! Too often people are substituting social media sites for websites. Facebook is fine if you just need a page for a group of people to talk and share a hobby. However, if you want to be taken seriously, and you are interested in being a business, then you need your own website.
For one thing, you will own your content. People may decide that they hate Google+ or LinkedIn tomorrow, and migrate to the latest and greatest interface. Where is all your content then? We have already seen this happen with Facebook and Instagram.
Plus, there are so many more possibilities available to you with your own website. There is incredible freedom to control your content, but just connect it to social media platforms. You write once and it instantly populates social media accounts.
There is NO social media platform substitution for the professionalism that is reflected by a real website. It shows your attention to detail and commitment to your product or service. If you are serious about social media marketing, then make sure you have a website.