Wiki spam is a form of link spam on wiki pages. The spammer uses the open editability of wiki systems to place links from the wiki site to the spam site. The subject of the spam site is often unrelated to the wiki page where the link is added. In early 2005, Wikipedia implemented a default "nofollow" value for the "rel" HTML attribute. Links with this attribute are ignored by Google's PageRank algorithm. Forum and Wiki admins can use these to discourage Wiki spam.
Good information you post here, but wiki spam is also black hat technique and to avoid it only Wikipedia implemented "nofollow". But this nofollow links are not useful in link building cause search engine spiders never crawl these pages.