As long as you are asking about using the sites, then yes you are on topic.
Using as a reader is definitely on topic.
Using as a contributor should be OK as you are still just using the software, albeit to create content. It's the same as, say, questions about uploading content to Facebook.
Using as an administrator is more problematical as this starts to get into areas of system setup and installation which are definitely off topic. However, some areas (setting up of user accounts perhaps) might be OK. I think we'd have to take these on a case by case basis.
See this question on the merging of the Wikis and Wikipedia proposal.