quite a deep question. To answer the first part of it: yes, social media is definitly important for F500 corps. Why? Because there are a lot of people on the social media platforms and the continue to grow. And while the social media platforms remain free (if you really only use the standard functions) every lead you pick up is free also. Like Zero bucks. 
Since a lot of companies are moving towards things like reputation retention/frontline customer care strategies, this person who spends the whole day on the web, could easily be involved in other online communities shoving out content, etc. 
One of the simplest things that come to my mind is to synch up your blog on several social profiles and thus automatically publish posts to the according websites. 
The second part: I am pretty much present everywhere. And has it helped me? Yes, it definitly has. I comment pretty frequently and participate in a lot of discussions, blog a lot of articles associated to my business but also about my private life. 

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