Thats fine if traffic is what you look to get from joining the site. Forums provide much more information so get my vote but as you say traffic will always be better at the social networking sites
you will not get the correct answer. you ask this on a social networking site and they will say social networks are good. you ask this on a forum and they will say forums are good. is it clear that some people are on social networks because they like it and some people are on forums because they like it.