Ok, so, the case studies cited in the article you linked:
Naked pizza (http://www.squaremartinimedia.com/twitter-business-case-study-naked-pizza/)
The only numbers they mention are these:
So what is the ROI for Naked Pizza using twitter? Well the company’s first twitter only promotion on April 23 resulted in 15% of total sales, with 90% of those being NEW CUSTOMERS! On May 29 twitter set a store sales record with the bulk of the traffic coming directly from twitter. How much? 68.60% of the total sales came from customers who stated “I’m calling from twitter”!
15% of total sales from twitter on April 23, total? counting brick&mortar sales as well? It doesn't tell how many twitter followers they have, what was the cost to collect those followers, they are not saying what is the CPA, the CR, they are not giving any metric to value if it was a real success or not.
In the same article when it talks about Dell case study is not linking to any real case study, but I did some google to search for one and I found this other article: http://www.informationweek.com/desktop/dell-makes-$3-million-from-twitter-related-sales/d/d-id/1080465?
It's rather old; it's state the big success of Dell have been 3 million USD revenue attributed to Twitter, from 2007 to 2009, with a total of 600.000 followers. I have no idea what is Dell average order value, let's assume it's 500 USD, it means a CR of 1% which is not all that bad, but we don't know the cost of generating those 600.000 followers.
Also when they refer to Whole Food case (http://www.socialmediaexaminer.com/reaching-millions-with-twitter-the-whole-foods-story/) they do not mention any CR or CPA or figure, Whole Food is saying they use Twitter and Facebook for customer care, they get in touch with customers, which is reasonable, but no sales. They got 1.7 million followers, which is an astonishing number, but they are a beloved brand, it's not hard to imagine they attract a lot of existing customers.
I still wish I could find someone with some figures showing a real success story.