I read it a year ago. It's a great book. Maybe I should read again.

Basically, the idea to solve the "cold start problem" (start a network effect from scratch) is to create extremely niched (but very active) communities, then expanding from that. It's like create each time a new and more broad concentric circle, addig more and more value for the existing communities (what causes more users wanting to join) and adding tweaking the product to repeat that for more communities.