Will this continued usage shorten the lifespan of the processors?
I think the PCs will probably be replaced long before there's any chance of the CPU dying of being worked too much.
And to what Steel said, I think the slipping of users may at least partially be attributed to the fact that the school year ended (at least, the college school year). I know I had mine going, being I lived on campus with free power and T3 and all. But now, I live in an apartment, and kinda refrain from keeping my box on all the time.
Though getting the P3 933s at work going is sure boosting me up there