Every day millions of users are connected through online social networks, generating a rich trove of data that allows us to study the mechanisms behind human interactions. Triadic closure has been treated as the major mechanism for creating social links: if Alice follows Bob and Bob follows Charlie, Alice will follow Charlie. Here we present an analysis of longitudinal micro-blogging data, revealing a more nuanced view of the strategies employed by users when expanding their social circles. While the network structure affects the spread of information among users, the network is in turn shaped by this communication activity. This suggests a link creation mechanism whereby Alice is more likely to follow Charlie after seeing many messages by Charlie. We characterize users with a set of parameters associated with different link creation strategies, estimated by a Maximum-Likelihood approach. Triadic closure does have a strong effect on link formation, but shortcuts based on traffic are another key factor in interpreting network evolution. However, individual strategies for following other users are highly heterogeneous. Link creation behaviors can be summarized by classifying users in different categories with distinct structural and behavioral characteristics. Users who are popular, active, and influential tend to create traffic-based shortcuts, making the information diffusion process more efficient in the network.

We study how to design experiments to measure the success rates of phishing attacks that are ethical and accurate, which are two requirements of contradictory forces. Namely, an ethical experiment must not expose the participants to any risk; it should be possible to locally verify by the participants or representatives thereof that this was the case. At the same time, an experiment is accurate if it is possible to argue why its success rate is not an upper or lower bound of that of a real attack -- this may be difficult if the ethics considerations make the user perception of the experiment different from the user perception of the attack. We introduce several experimental techniques allowing us to achieve a balance between these two requirements, and demonstrate how to apply these, using a context aware phishing experiment on a popular online auction site which we call "rOnl". Our experiments exhibit a measured average yield of 11% per collection of unique users. This study was authorized by the Human Subjects Committee at Indiana University (Study #05-10306).