The Web is an unsteady environment. As Web sites emerge and expand every days, whole communities may fade away over time by leaving too few or incomplete traces on the living Web. Worldwide volumes of Web archives preserve the history of the Web and reduce the loss of this digital heritage. Web archives remain essential to the comprehension of the lifecycles of extinct online collectives. In this paper, we propose a framework to follow the intern dynamics of vanished Web communities, based on the exploration of corpora of Web archives. To achieve this goal, we define a new unit of analysis called Web fragment: a semantic and syntactic subset of a given Web page, designed to increase historical accuracy. This contribution has practical value for those who conduct large-scale archive exploration (in terms of time range and volume) or are interested in computational approach to Web history and social science. By applying our framework to the Moroccan archives of the e-Diasporas Atlas, we first witness the collapsing of an established community of Moroccan migrant blogs. We show its progressive mutation towards rising social platforms, between 2008 and 2018. Then, we study the sudden creation of an ephemeral collective of forum members gathered by the wave of the Arab Spring in the early 2011. We finally yield new insights into historical Web studies by suggesting the concept of pivot moment of the Web.