Direct link to article: http://www.techrepublic.com/article/the-death-of-ruby-develo...

A professor is not a good expert for this.

How come it says the popularity of Ruby has plummeted?

According to the article they're basing it on job postings and stack overflow questions. Doesn't seem like a great methodology but I can't think of a better one.

StackOverflow already covers a great part of the possible (most common) Ruby related questions. I'm not surprised there aren't so much new ones in the last 1-3 years. About job postings, I don't know, I still get an average of 3-5 ruby job opportunities per week from recruiters. The same goes for several ruby devs I know. On the other hand, I see a lot of people switching to other new languages/stacks for several reasons (some of which are mentioned in the article). I'm one of them, but I still use ruby when it might fit the job (or is a requirement).

I looked at TIOBE and Ruby was up year-over-year.

People have been saying this for 5 years.