To gain insights into the evolution of social networks, in this paper, we study social networks constructed from the relationship of reposting and being reposted in Sina microblog, the most popular social networking website in China. We focus on a case of protecting plane trees in Nanjing, China. The event was aimed to protest Nanjing government for transplanting thousands of plane trees with superb historical value, for the purpose of constructing subway stations. The event lasted 8 days, culminating in a mass parading on the 6th day, and a subsequent compromise by the Nanjing government of leaving the remaining plane trees un-touched. Interestingly, we find that the network based on “reposting” is a scale-free power-law network. In contrast, the network based on “being reposted” is in general not a power-law network. Rather, the degree distributions there follow the celebrated Omori's law for earthquake after-shocks, suggesting that powerful microblog postings were like strong earthquakes that may elicit a sequence of after-shocks, i.e., numerous further postings and discussions. The parameters for the power-laws and Omori's laws changed from day to day, with the 6th day being totally different from other days, coinciding with the fact that mass parading was on the 6th day. Our study thus shows that much can be learned from the study of dynamical evolution of social networks.