You can try to blame FB for taking up the free time a person might have used chopping for contests, but PSC has changed a lot and in several stages over the years; it simply hasn't kept the same draw to all the same people. In other words, PSC has been "dying" long before FB gained much popularity around here. And yet, you will still find hard-core PSC addicts. Just because many are relatively new and not among the "founding members" does not make them any less vital to the site.
My own addictions to chopping and chat have taken a backseat as I've taught myself crochet, built a hen house, and played with my little girl. For me, Facebook actually had nothing to do with my becoming less active on PSC, but eventually did offer a way for me to meet up again with other "semi-retired" PSCers. At least a few of us actively follow both sites to one extent or another. I have used FB to help get the word out about different events on PSC, and I've had several PSCers thank me for reminding them.
