I think it would create more user-interaction in the forum, if the user gets frequent updates sent by e-mail. There should be an option at which the user can check whether he wants to get an e-mail when there are any news/updates/posts from the people he is "following". Probably there are other ideas to this, but I can currently of just sending an E-mail to the user related to the "Your Newsfeed". This would make sense. I see that Linkedin.com is sending me weekly updates named "Network Updates" which is showing some posts from certain people.