In regards to corporate newsletters, this post was just as ridiculous. Twitter likely had nothing to do with the decline. He has absolutely no evidence indicating it in any way, other than twitter growing while the newsletter ctr dropped. Nearly everyone on earth gets email, and has an email account. Twitter's audience is minimal in comparison. But rather than say anymore, I'll just refer you to maxklein's comment above. He really nailed it.
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The article here though does seem to show that a more focused newsletter can still be quite useful/valuable:
http://news.ycombinator.com/item?id=865042