1. In a sense, finding cofounders seems like the wrong problem. I mean the goal is to have customers and finding a cofounder is often more like checking off a box on a Ycombinator application than actually doing the hard work of building a product that someone might look at and want to team up on.
2. That's not to say that having cofounders isn't a really good idea. Running any business, nevermind a startup, is often a rollercoaster and doing it alone is going to be harder for many people to the point that it vastly increases the odds of not succeeding.
3. I don't know about Cofounderslab, but Angellist is a social network and therefore more about connecting people who are already connected in the graph with other people in the graph than about creating lots of connections for newcomers. Or to put it another way, if technical person A is connected on Angellist to investor B and investor B is connected to investor C and technical person B is connected to investor C, then Angellist facilitates hooking up A and D. And if business guy E is connected to D, then maybe D's in the mix too. But if I join and don't know anyone, Angellist is not a lonely hearts club.
4. How often does anyone look for a cofounder? Once every...how many years? That's why something like Angellist is investor driven: investors are looking for deals with a high frequency and participate in multiple deals in parallel. Investors are also where the money is and where the money is is where people who need money will tend to go.
In the end, finding cofounders is a real problem for some people, but it's not one that a website dedicated to solving it is likely to solve. Mainly because it's not a problem at a large scale, partly because it is a problem that is only solved one person at a time.
Good luck.