Google+ still hasn't taken off in the UK, most users are finding it complicated to use at the moment. Facebook is good for designers, retail leaders and pop culture, as is twitter. Linkedin is amazing for business to business, youtube is obviously the only choice for video media, other forms of visual media would be better off on pinit, then there are networking sites such as thebizniz which also work extremely well.
So to be honest, considering all the successful business people I've met use people in every sector, it would be better to get on as many as you can, that way you can increase your customer base, and potentially reduce cost at the same time. The only downside, is this amount of time it takes out of running your business.