In my opinion, the social media site which will best yield you conversions should be used for the business.
For eg. If were a local business, I would focus on foursquare, yelp, Google Plus
If I were a fashion or food blogger, I would probably go for Pinterest or Instagram
If I were a B2B business, then Linkedin would be the go to platform.
The important thing to understand here is that focus on who is your target audience and where they are concentrated at.
For eg. Pinterest won't be a good fit for a plumbing company because your audience doesn't reside there.
Its necessary that you test-test-test. Do your research, talk to your current audience, learn their interests, likes and then begin focusing on platforms that will best bring in conversions for you or your clients.