This is a couple of years old but it's the first time I've seen it. No surprise for any of you who've ever participated in an online community, online course discussion board, looked at Wikipedia...
"In most online communities, 90% of users are lurkers who never contribute, 9% of users contribute a little, and 1% of users account for almost all the action.
All large-scale, multi-user communities and online social networks that rely on users to contribute content or build services share one property: most users don't participate very much. Often, they simply lurk in the background."
See the whole article for ideas on ways of better equalizing participation.