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