I just read on a blog about how Fox News had a blogger and this blogger developed a strong following. The importance about this blogger was that the visitors to his blog also visited other pages of Fox site and this is what content publisher sites really desire - to increase the stickiness value of site visitors. But guess what? When the blogger was dismissed, the bloggers visitors did not return to the site. Thus, this brings up an important question - who owns the social media conversation when someone leaves a company?
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I think the example you pose shows one side of it. What it comes down to is the 'product' the blogger or the site? Some bloggers develop followings and when they leave a site it would stand to reason that a good majority would leave the site. But by the same token, it is probable that some of the bloggers loyal following would find other content on the site of interest and still return but not as regularly as when the blogger was there.

So I would say the answer to your question, in my humble opinion, is it depends. Some bloggers will be more powerful than the site and some sites will be greater than the sum of their contributors.