Updated Nov, 11th 2012
I think they need to go back to all their users (both sides) and understand why this is not a MUST HAVE product. If they have under 40% people saying they would be dissappointed if it went away then they might need to understand what they are missing in their product design or user experience.
I would want to know the following:
1. what engagement metrics they are using?
2. Why do people come back and what are they using as content or stimulus to get people to come back to the site?
3. Are they creating a community to keep engaging people so new deals are submitted that are quality and showing success stories of the people who got funded by being a member? or investors who made money and their investing into certain startups they found on this site. Sounds like they are doing a poor job of communicating the value of being a member and returning. Its kind of like playing slot machines. If there is no motivation to come back then people only play once.