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 disappointing, 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.
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.
Deeper analysis
See Green wikis and development wikis. The table can be sorted by levels of activity, numbers of pages and other criteria by clicking on the headers.