That may all end now that they've been acquired by a mega corp though.
Look at this trend: http://trends.google.com/trends?q=okcupid%2C+plentyoffish...
Kuro5hin was never as popular as Slashdot.
Slashdot didn't evolve their submission model and was left in the dust.
Digg screwed up their product and pissed off their community.
MySpace sacrificed user experience for in the name of short term profit from pageviews/ads.
HN hasn't replaced Reddit except for a tiny minority of startup/hacker people (many of whom continue to use Reddit).
I think it only appears that users are fad driven, when in reality they're just smart enough to move on to better options.
On the other hand, I doubt that reddit isn't going to suffer the same fate that slashdot and digg did when something better comes along, though I doubt they are going to make as drastic a mistake as digg did.
My guess is googling for "okcupid" all as one word is relatively more frequent than "plentyoffish" as one word, and here's confirmation using Google Insights for Search, which allows OR queries:
http://www.google.com/insights/search/#q=%28okcupid%20%2B%20...
As much as the PoF guy's pontificating gets old to me, and agree with your Facebook-MySpace analogy, he's still killing OKCupid traffic-wise.