Customer Communities - Utilization, Traffic and Engagement - what works?

Interested in hearing from those of you who have set up online Customer and/or Partner Communities. I've been looking at the traffic patterns for many community sites and it often appears that companies are able to drive an initial spike of traffic when the Community is first launched (probably owing to the curiosity factor of customers wanting to 'check it out') - but in many cases then see traffic fall off quite significantly beyond that point. Have you experienced this? What does your company do to foster ONGOING engagement and participation in your community(s)? Also would love to hear from those of you who have opened up your communities to non-customers, and particularly any companies that have used tactics such as search marketing to bring traffic and participation from outside your existing customer base. How has that worked out?