What all these sites have in common is that, despite being free to access and offering free cloud facilities (Linkedin also offers a premium paid-for service), they are first and foremost commercial. They strive to attract lots of people in order to provide a receptive audience for advertisers and other commercial interests. Further, they encourage their members to divulge personal information that enables businesses to target their audiences.
Opinions about this information gathering vary. Some see it as sinister, some as a reasonable price to pay for the free services. Others regard it as a positive way of bringing useful information about products and services to their attention.
All these platforms offer businesses their own sparate identities. Facebook has its 'pages', Linkedin its company profiles, and Google Plus is now following suit. Again these are basically free, but there's lots of encouragement to spend on adverts and other premium services.
If you add other, more tightly focussed platforms, such as Twitter, Tumblr, and the up-and-coming Pinterest, into the mix, you have quite a bewildering spread of options. Where do you devote your main energies? Can you afford to just ignore some of them? These questions have answers. What the answers are depends very much on the particular nature of your business, and it might take a little research and experimentation to find out where your audience is hiding.