So Y asked three interesting questions regarding Grid-Group Cultural Theory. This is a line of thought, a method of inquiry, developed by the British social anthropologist Mary Douglas, along with numerous collaboraters, and more recently numerous younger adopters who never actually worked with Douglas. Its early presentation was in the influential book Natural Symbols.
DMK has already given a response to this in the original comments (many thanks!), and here’s my additions.
1. is the theory considered to be a post modern one?
Quick answer: no. Slightly longer answer: The theory was developed on the cusp of the rise of the postmodern as a dominant category of analysis. Neither Mary Douglas nor Aaron Wildavsky were involved with anything that would be recognisable as explicitly ‘postmodern’. Like Derrida, Douglas was strongly influenced by the work of Claude Lévi-Strauss. But whereas Derrida subverted structuralism, Douglas extended it. In particular they each took quite different approaches to Levi-Strauss’s methodological use of the distinction between nature and culture. In many ways Cultural theory might appear to advocates of the postmodern as hopelessly compromised by the ‘grand narrative’ that there are four and only four cultural worldviews. That’s what I like about it. On the other hand, there are many, I think, who see the ‘constrained relativism’ (Marco Verweij) of Cultural Theory as being too relativist for comfort. I like that too.
For more context, Richard Fardon’s book is invaluable: Mary Douglas: An Intellectual Biography.
2. does it have prestige in the academic world or is considered niche theory?
I think it has some prestige, but precisely as a niche theory. For example, in the study of risk, CT is one of three main approaches to the subject, but only one. In social anthropology it would probably be seen as obsolete. Fardon’s book has a section entitled ‘Theoretical Marginality”. However, it’s quite possible to make an academic career out of Cultural Theory (or a re-branding of it) and a number of highly respected academics have adopted or adapted a CT approach for at least some of their work. But there is no large movement or institution that has adopted it as a significant approach. CT’s strength/weakness lies in that fact that it has been applied piecemeal in a large number of different disciplines. It appears to have an explanatory power as yet not fully realised. I think the conceptual strengths of Cultural Theory have not really been matched by its methodological capacity. There is potential to further develop rigorous methodologies that develop some of the concerns of Cultural Theory.
3. do you think that online/virtual communities on the internet can also be classified according to the grid group (like wikipedia, linkedin etc)?
Yes. Prof Sun-Ki Chai, at the University of Hawaii is a very rare individual in that he has both published on Cultural Theory (he edited a book of essays by Aaron Wildavsky, I believe) and patented a web crawler that can analyse web data according to several social science approaches. His work shows a way to do what you suggest, from a predictive social science angle.
Related articles
- Some questions about Grid-Group Cultural Theory (fourcultures.com)
Four Cultures 
I agree that Sun-Ki Chai is an excellent, & multifaceted, scholar. He has written on the use of attitudinal scales based on CT for cross-cultural studies of cognition & has at least one published study that uses cultural theory classifications to assess behavior in experimental game theory (Chai, S.-K., Dorj, D., Hampton, K. & Liu, M. The Role of Culture in Public Goods and Other Experiments. PS: Political Science & Politics 44, 740-744 (2011)). I was not aware that he had developed a method for characterizing on-line communities with Group-Grid — it’s to learn important things like this that I read Fourcultures blog!
I would guess that Grid-Group Theory would predict that it would not itself be widely accepted.
In fact, I belive that if it WERE widely accepted, then that would prove it false.