netbuzz writes "In an attempt to thwart spammers and search-engine optimization mischief, Wikipedia has begun tagging all external links on its site "nofollow", which renders those links invisible to search engines. Whether this is a good thing, a bad thing, or simply unavoidable has become a matter of much debate." This topic has come up before and the community voted to remove nofollow back in 2005. This new round of nofollow comes as a directive from Wikia President, Jimbo Wales.
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