Apparently, the Stack Exchange community is not cited in the Wikipedia article "List of virtual communities with more than 1 million users".
Is there a special reason for that or should it be added?
I don't see any reason it should not. Just Stack Overflow has currently about 5.6 millions users and the whole stack exchange community is even bigger. I'm not sure how to count the total number of different users over the whole network but it is certainly more than a million.