The relationship between Web 2.0 and social media marketing is simple: from a website owner’s perspective, Web 2.0 is important because of its focus on user submitted and user controlled content that many Web 2.0 services and sites allow.
In general terms, when people can make comments, submit content or links to content, and rate or vote on content, it affects the amount of traffic that content can generate.The visitors take that control of what shows up in the most prominent places by voting, commenting and other actions can literally make or break a marketing campaign.
As a marketer, being aware of the best places to show up and how they work will bring you traffic. More traffic than you ever got without social marketing in many cases.