Online social post streams such as Twitter timelines and forum discussions have emerged as important channels for information dissemination. They are noisy, informal, and surge quickly. Real life events, which may happen and evolve every minute, are perceived and circulated in post streams by social users. Intuitively, an event can be viewed as a dense cluster of posts with a life cycle sharing the same descriptive words. There are many previous works on event detection from social streams. However, there has been surprisingly little work on tracking the evolution patterns of events, e.g., birth/death, growth/decay, merge/split, which we address in this paper. To define a tracking scope, we use a sliding time window, where old posts disappear and new posts appear at each moment. Following that, we model a social post stream as an evolving network, where each social post is a node, and edges between posts are constructed when the post similarity is above a threshold. We propose a framework which summarizes the information in the stream within the current time window as a ``sketch graph'' composed of ``core'' posts. We develop incremental update algorithms to handle highly dynamic social streams and track event evolution patterns in real time. Moreover, we visualize events as word clouds to aid human perception. Our evaluation on a real data set consisting of 5.2 million posts demonstrates that our method can effectively track event dynamics in the whole life cycle from very large volumes of social streams on the fly.