When you go invite only, your new memberships start dropping fast. Think about how many of our members have met another member before they joined the forum. The forum already has one of those letters/numbers in a fuzzy image things that's supposed to deter spam, but obviously it's not stopping all of it. Mods approving new posts is IMO the best way, but it only works if you have enough mods to keep up with it, but right now AFAIK it's just Teran pulling mod duty, and that's really not fair for him.