Get the activity on your forums/blogs/whatever out in front of the users!
Have you ever seen the "recent posts" hacks on vbulletin.org that so many forums rely upon? Their success is because users want an easy way to see all of the updates that are made to a given site.
Forum? Blog? Social Groups? Directory? Wallpaper?
All of this is bunk unless the users of a site know what's been updated, at the moment. Facebook, anyone?
So you can write your forum script. You can try to compete with vBulletin, SMF, and others.
But what you need to concentrate on, first and foremost, is the mechanism whereby you put in front of people's faces the updates that are made in the various areas that you code!
Otherwise, your efforts are farts in the wind.