I don't think the sign up to enter is bad, it works if done well and users who are willing to sign up are more active but the problem I see on the golf site is you only have 2 small lines at the end of that form telling people what the heck the site is about.
People are suddenly being presented with this huge sign up form before they even know what they're signing up to join.
As far as social site for snowboarding, I don't see it working. You can have social features as an add on to a site about something else but if the main focus of the site is just to get people to sign up for 'Facebook for snowboarding' it wouldn't work.
Honest I think the bigger problem with niche social networks is people don't really want another Facebook when all their friends are already on Facebook. It's far easier to setup a Facebook group and invite your friends there then trying to get them to join a whole new social network.