Will my methods chapter be less solid, if it is based on one book or the work of one scholar?
Literature on virtual ethnography or netnography is not restricted only to these authors. A cursory Google Scholar search reveals many other authors. 

Having said this, ~50% of my methodological approach in my work is through grounded theory by using semi-structured interviews. The gold standard for grounded theory is:

Glaser, Barney G &amp; Strauss, Anselm L., 1967. The Discovery of Grounded Theory: Strategies for Qualitative Research, Chicago, Aldine Publishing Company


... and I would wager that ~95% of papers using grounded theory cite only them in the methods sections of their papers.

This does not necessarily make your methods poorer or stronger. You should make sure that you understand the methodological process perfectly and then implement it. 

Make sure that you do talk about credibility, transferability, dependability and confirmability as well !