I think the problem with these founder dating type sites is that starting a company really isn't like "dating" at all, its more like a marriage. It's tough to just meet someone on a whim and dive into something like starting a company. You wouldn't just walk down the aisle after meeting someone on an online dating site and exchanging a few messages or going on a few dates either (hopefully), and you probably shouldnt launch a company after a few beers with someone you met on a website. These sites can introduce the two of you, but its on you to determine how well you will work as a team.

I'm not really sure how you solve this problem, or if sites like CoFoundersLab can solve the problem. I've met up with a few people from these sites for drinks and coffee to bounce ideas around, but nothing really went further than that. IMO, The best thing to do for those actively looking for a cofounder is to constantly be "out there" as pointed out by quantisan. That means leveraging your network, going to meetups...etc and being active with it.