I would never use a site to find a co-founder. It seems like a waste of time. How would you vet someone? There is so much noise in the space and these kind of sites attract a ton of people who are not prepared/knowledgeable to found a successful business. I will only ever co-found with people who are part of my network and I know well enough already. Usually people who I have had some experience working with already. From time to time I have tried to be more loose in my approach but it has never led anywhere.