Argumentative discussions are common in Web 2.0 applications, but the social Web still offers limited or no explicit support for argumentation. As Web 2.0 applications become more popular, modeling argumentation happening in these systems becomes important, to enable reuse and further understanding of online discussions. After reviewing four genres of online conversations – Web bulletin boards, Wiki talk pages, blog comments, and microblogs – and four current Web 2.0 argumentation systems, the paper suggests how Semantic Web technologies can be used to provide an interoperability layer for argumentation modeling across applications.