First, tell me the number where the world became overpopulated? 5 billion? 3 billion?

You can calculate a number from the resource (over)usage.

Some will, of course, claim that we consume too much – but at that point, it's a matter of opinion. I call it overpopulation for the current level of technology.

That's right. Overpopulation is a function of technology, etc. More people are coming out of poverty, and according to Bill Gates there will be almost no poor countries by 2035:

http://www.cnbc.com/2014/01/21/bill-gates-they-will-be-no-po...

If we can't figure out how to generate electricity by means other than coal, for example, we will have a problem. However, technology is increasing rapidly and increasing the quality of life for the majority of people is happening on a large scale.

http://www.businessinsider.com/chinas-middle-class-is-explod...

http://www.worldbank.org/en/news/feature/2012/11/13/middle-c...