I think the 'Connect with your social network account' option should be advertised more prominently - right above/below the login fields if possible.
The easier and less commitment-requiring it is for users to register and participate, the faster the forum will grow IMO. I was not going to register until I specifically searched out for the 'connection with a social network account' option, and found it. I had visited the site a few times, took a quick glance at the login fields, and not seeing any 'connect with social network' option, abandoned the idea of registering.
Also, after registration with the social network account, please provide a message saying "you will need to click on the activation link sent to your email address in order to login with your social network account', because after registering, I tried a couple times to login until I figured out why it wasn't working and activated my account.
Also, do you need to require users to click an activation link after they've registered with a social network account? Is it really necessary when they've already proven to have an active social network account? It seems like an unnecessary step to registration/activation.