Do an internet search for a wiki on a particular topic, e.g. permaculture wiki or sustainability wiki and you'll see how many sites there are. But when you click on them, most will be disapponting, a few will be interesting, and the occasional site will be a fantastic resource.
However, most wikis are dormant, while yet others are spammed. A handful have a small amount of activity, and a very small number are vibrant, with an active community creating a growing knowledge resource.
each site represents an island, a small isolated community and a fragmented knowledge base.
See also Green wikis and development wikis - a more detailed comparison can be found there. Search the page (ctrl-F in most browsers) for permaculture and use the sortable table on that page to find the most active wikis, or the wikis with the most pages. Some great efforts have been made; however,
Why is this? Because making a successful wiki is hard.
What's a better approach? Radical collaboration. How do we do this for wikis? Practice Wiki synergy.