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The global feed sorted by time gives a pretty good idea of post activity. But we just had an influx of new people. You could compare the entire site's activity to a middle-sized subreddit. However, commenting and post activity seems to vary in waves. None of the individual tags is active enough to be considered a stand-alone community. #music is the most active tag. You can see all the tags sorted by various metrics on the tag page (link also in the upper right).