Abstract-Reputation mechanisms are essential for online transactions, where the parties have little prior experience with one another. This is especially true when transactions result in offline interactions. There are few situations requiring more trust than letting a stranger sleep in your home, or conversely, staying on someone else's couch. Couchsurfing.com allows individuals to do just this. The global CouchSurfing network displays a high degree of reciprocal interaction and a large strongly connected component of individuals surfing the globe. This high degree of interaction and reciprocity among participants is enabled by a reputation system that allows individuals to vouch for one another. We find that the strength of a friendship tie is most predictive of whether an individual will vouch for another. However, vouches based on weak ties outnumber those between close friends. We discuss these and other factors that could inform a more robust reputation system.

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Abstract.What are the benefits and drawbacks of integrating health and wellness interventions into existing online social network websites? In this paper, we report on a case study of deploying the Three Good Things positive psychology exercise as a Facebook application. Our experience shows that embedding a wellness intervention in an existing social website is a viable option. In particular, we find adherence rates on par with or better than many other Internet-based wellness interventions. We also gained insights about users' privacy and audience concerns that inform the design of social networkbased wellness applications. Participants did not want all of their entries to be shared with all their Facebook friends, both because they did not want others to know some things and because they did not want to clutter others' newsfeeds. Users found it compelling, however, to interact with their friends around some "Good Things" they had posted.

As online social networks expand their role beyond maintaining existing relationships, they may look to more faceted ratings to support the formation of new connections between their users. Our study focuses on one community employing faceted ratings, CouchSurfing.org, and combines data analysis of ratings, a large-scale survey, and in-depth interviews. In order to understand the ratings, we revisit the notions of friendship and trust and uncover an asymmetry: close friendship includes trust, but high levels of trust can be achieved without close friendship. To users, providing faceted ratings presents challenges, including differentiating and quantifying inherently subjective feelings such as friendship and trust, concern over a friend's reaction to a rating, and knowledge of how ratings can affect others' reputations. One consequence of these issues is the near absence of negative feedback, even though a small portion of actual experiences and privately held ratings are negative. We show how users take this into account when formulating and interpreting ratings, and discuss designs that could encourage more balanced feedback.

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