Research has shown that the opportunities for social loafing increase with the size of a given group, as members perceive their contributions to be less significant to the overall group and therefore start to participate less.  How can researchers avoid this when running a private online research community?  Here are a few tips for avoiding social loafing in MROCs:

1. Keep it small - Even in a group as small as an in-person focus group you can start to see social loafing occur, as certain participants will rely on the most talkative participants to make their way through the two hour group.  You can imagine what happens when you increase the size of that focus group to a couple hundred or even thousands of people.  You end up paying a lot of money recruiting and providing incentives for people to merely lurk and loaf off the contributions of others.  Keep your MROC on the small size and you'll be spending less to get more.  We like 150 participants for ongoing MROCs, but experiment with the size that works best for your organization and objectives...  
2. Make the shared background and purpose transparent to members - Research has shown that people will socially loaf less and contribute more to a group the more they like it.  Social psychologists have also noted that people tend to like other people that are similar to them.  Given this, try to create communities that share as many commonalities as possible.  Doing so will help encourage contributions above and beyond what you might see in a "generic" online community.  The nice thing about MROCs is that members are specifically recruited according to various demographic/psychographic attributes, so researchers have a real opportunity to craft commonalities among members.  However, divergent backgrounds and opinions shouldn't be ignored, as conversations among different types of members can also produce interesting ideas and insights.  
3. Increase the level of moderation - Have the community moderators spend as much time as possible acknowledging individual contributions to the group through frequent probes.  Even if the probes don't directly contribute to the research findings, you will still be creating opportunities to acknowledge individual contributions.  Having a points system in your platform can also help to "tangibly" acknowledge individual contributions.  For example, the PluggedIN Platform has the capability to reward members with additional points on individual comments or user generated content, for those times when you really want to highlight member contributions to the research objectives. 

Social loafing can be a real problem in online communities, particularly as the size of the community grows.  However, taking some of these steps into mind should (hopefully) help mitigate the impact of social loafing on your next MROC.

-Matt