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I have none. 3-5 is a good idea I guess.
I would say 0. It's very rare that a potential client will want to follow you on any SM platform until after they know/have met you. So, it is just creating clutter. Makes it less likely for them to click on your website (which is much higher priority). Furthermore, it is draining your link juice and page rank. So I say 0.
I may have three.
Stick to 4 only - Facebook. LinkedIn. Twitter, G+
Kevin Mackessy , I think Debbie Gartner is correct. I'm not sure anyone is interested in following my blogs.