Fig. 6

The density distribution of active users with different proportions of three types of activities: post, share, and reply in each period. The direction of the tick label (e.g., 20, 40, and 60) indicates that the cells on the same direction of the tick label represent the same value of the type of activity. The proportion of users for a certain type of activity remains the same along the direction of each tick label. For example, the direction of the tick label on the side of “Reply” is horizontal, which means the cells horizontally correspond to the tick labels. Specifically, the proportion of the replying activity is very high on the top of the triangle, while the proportion of the replying activity is very low at the bottom of the triangle