Apparently, I come across as one. I detest the term and don't believe it is a well-posed construct. People who call me that imagining they are giving me a compliment should be aware that I view it as an (unintended and forgivable) insult (sorry Joshua Engel, but I had to be honest here; I have no idea why you like it).

The famous names cited in the question weren't polymaths. They ranged across modern categories and boundaries that didn't exist when they did their work. In their own times, they were monomaths (to coin a term) than polymaths. Polymath is much more impoverished concept and it is somewhat insulting to apply the term to them. They saw beyond categories rather than through them.

I don't use it as a self-descriptor. I cringe when somebody applies it to me and usually try to talk my way out of that particular pigeonhole.

Is Quora full of them? If the construct is not well-posed, it is not a meaningful question. What you actually see is a distribution across a few other better-posed categories that can come across as "polymath." Categories like:

1. Erudite
2. Wannabe grand unifier
3. Idiot
4. What Howard Gardner calls "lumpers" (incompetent idea connectors/generalizers who add overhead rather than insight)
5. Opinion-on-everythingers
6. Intellectual gluttons: http://www.ribbonfarm.com/2010/0...
7. Refactoring types (people who are actually very narrow, but appear broad because their operating categories are very different). I self-classify into this.

Mostly, they are like Trollope's character Everett Wharton.

[He] had read much, and although he generally forgot what he read,
there were left with him from his reading certain nebulous lights,
begotten by other men’s thinking, which enabled him to talk on most
subjects. It cannot be said of him that he did much thinking for himself
— but he thought that he thought.