Collaborative Projects : Wikis and the biggest Wikipedia.com
Blogs and Microblogs : Wordpress and Twitter
Video Communities : Youtube and Dailymotion
Social Networking Sites : Facebook and Myspace
Social Bookmarking Sites : Stumbleupon, reddit and Digg
Virtual Worlds and Games : World of Warcraft etc.
For Marketing you should make use of Blogs, Videos, Social Bookmarking and Social Networking Sites.
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