Can answering questions on Stack Exchange sites be called volunteer work?

Solution 1:

Volunteer work, being charitable, requires abstract intent to do the good without expecting a direct personal gain except for the self satisfaction feeling which could be worth more than money to some people.

I think this applies to the professional participants who have the real capacity to answer questions correctly.

On the other hand, the non-professional participants should be classified as learners not volunteers (although they may sometimes have valuable participations).

Solution 2:

Can the term "volunteer work" be applied to answering questions on Stack Exchange or Stack Overflow trilogy sites?

I suppose so, since it's work, you are not being paid for it and are not obliged to do it, but most people take volunteer work to mean work done for a charitable cause.

Solution 3:

If you read one of Daniel Pink's books for eg. Surprising Science of Motivation, you would discover that an intrinsic motivator of a sense of mastery, autonomy and purpose drive us all to contribute here.

The reputation scores are extrinsic motivators that certify the sense of mastery among peers. I feel that anything driven by something that pleases you in some way is not volunteer work. It is certainly not charity. This is true even if you are a professional.