I'd never heard of any of those sites, I checked them out none made me want to sign up right now.
I clicked through on okcofounder here wanting and expecting to be able to sign up only to be disappointed.
Ideas are essentially worthless without effective implementation. This roughed out one page implementation was effective for me and I'm clearly not alone.
Well at least he has the best name out of the ones listed. Am I missing something though, his site seems to just be a description of what he's looking for a link to a contact form? Do we consider this a product now?
You're right. But the fact that he received so much disappointment so quickly means that those venues aren't working - yet...so yes, room for innovation.
I really, really want to reiterate Recoil's point above.
The rental market is massive and real and worth a shit-ton of money. Handling the pain of repairs and stuff? it's not even been vaguely tapped! That's a fucking good idea that's going to be hard to sell but it's a domino thing. When one major rental agency starts using it they'll all start using it. 'Your flat doesn't have residee.ce? How lame!'
Finding a co-founder is a tiny market.
Your advice is well intentioned and dog-fooding and all that lovely bullshit nonsense and really, really terrible.