In the course of our day we field a fair number of questions about research communities.  Here are three of the most frequently asked questions we get about market research online communities here at PluggedIN, along with our thoughts on each...

1. What is the ideal number of participants for a research community?  This is something of a debate among researchers, and there really is no "right" answer.  Our philosophy is that smaller (around 150 participants) is better, since it helps members get to know each other better and is manageable to moderate/analyze, while still allowing for a wide range of responses to each of the activities.  However, some advocate really large communities (on the order of hundreds or thousands of participants) in an effort to combine qualitative and quantitative methods.    
2. How long does the typical research community last?  We've found that 3-6 months allows you to cover a wide range of topics, while still allowing time to explore specific areas of interest in-depth.  However, communities that are built more as ongoing advisory boards can last for years.  In addition, communities can be used on a very short-term basis (e.g., 1-3 months) as a way to augment or replace other research methodologies like focus groups or online bulletin boards. 
3. What is the recruiting process like for a research community?  Recruiting for an online research community is very similar to other forms of online qualitative research (or even in-person methods).  While an online survey to screen for participants is fairly common, telephone recruiting and recruiting from in-person research studies are also viable means of screening participants for your study.  Sample sources may include client-provided sample, links posted on various social networks and online communities, panel sample providers, etc...