Millions of web users engage in the online activities such as blogging, online forums, or online review systems to interact with people and to capture attention. This study tries to understand how online users compete for the scarce resource, attention, when participating in the online Web 2.0 activities. We develop a framework to capture the decision making process for online users to choose a right topic to post information and right content to post. Using book reviews from Amazon, we find that online reviewers do behave rationally in order to gain attention and to enhance their reputation. Our results suggest that experienced or top ranking reviewers are more likely to review relatively obscure books to avoid severe competition for attention in popular books. Moreover, top ranking reviewers usually post reviews earlier than low ranking reviewers as there are fewer reviews coexisting at the early stage to compete for attention. In terms of review ratings, we find that low ranking reviewers post more extreme ratings which distance themselves from the current average rating.