The main problem I see with building such a community is that the people who return to it aren't the kind of people you'd want to have as cofounders. All the good candidates will form startups and get on with it and no longer have a need (or the time) for such a community.
As for a good way of finding cofounders, I can't really answer that from experience. I've done a poor job of doing that myself having not found any, but I'd rather go it alone than just team up with someone I've randomly found on the internet.
If you've got a bunch of friends who are kind of on the fence about the whole thing, either because they don't like your idea (big red flag!) or because they're not sure they want to commit to doing a startup, I recommend talking to them and forcing them to be honest with you about your idea and their plans. You could see if they'll join you after you've built some kind of proof-of-concept/prototype on your own, although I recommend you plan for the case where they never follow through with it.
If your friends just plain don't want to do the whole startup thing (that would account for 100% of my hacker friends) and you decide to go it alone, try and keep a few of them in the loop of what you're doing as much as possible. They won't replace a good cofounder, but better than nothing and will keep you grounded.
By the way, 'friend' here refers to people you have a lot of trust in, as opposed to 'acquaintances'.