Another point in function over design, just like Craigslist.
PoF is successful in spite of its questionable design, not because of it.
So poor usability actually is a feature.
Which comes back to the old argument about advertising-driven revenue models resulting in poor user experiences, since the "user" from a revenue model is the advertiser and YOU'RE the product.
Even with that factor though, I found PoF borderline unusable after a little experimentation and stopped using them in favour of OKC. I can't believe I'm the only one like that, or that there aren't some who simply dropped off PoF into nothing.
If that crappy site can extract value at such a high level, I'm sure a few reasonably small tweaks could extract significantly more.
At the scale pof operates at you have to A/B test everything because some things are very counter-intuitive.
My favorite anecdote of his (tried to find it on his blog but google is failing me) was when he briefly fixed the problem I hated most about the site: the aspect ratio of the profile pic thumbnails being off.
What happened is that it cost him lots of money. And the explanation as to why makes sense too: when the aspect ratio is messed up people can't tell at a glance what the person looks like so they click through to people's profiles a lot more instead of just scanning the top bar/search page. When your business model is based on page impressions getting people to load more pages means you get more money.
So in that case at least it's better for his pocket to have something that's obviously broken (and easy to fix) on the site.
Now, I could make the case that PoF has ridden the wave for long enough to build Markus enough money that if it all fails tomorrow he's still made for life and only really lost pride, but....
Wasn't that what I said?