Hi Verena @vkick
I think that the collaborative nature of the site-wide wiki does suggest a default Creative Commons Attribution NonCommercial Share Alike 3.0 license – but it largely depends on the nature of your site. Also, groups might have private wikis and might choose to license content differently by marking it with a copyright sign, name, and the year.
Here is a link to the CUNY Academic Commons Terms of Service – http://commons.gc.cuny.edu/about/tos/ – you might want to read sections b and c.
Maybe others on this forum have ideas about this too.
Hope this helps.