Change in activity tends to snowball, whether for the good or the bad. If people post less, there are less posts to respond to, which means people in essense post less and less.
If people post more, there are more posts to respond to, people post more.
It's not that hard to understand, and if you've been around fairly consistantly for a few years you should see this yourself. Luckily, we've been going upwards lately.
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