There is some very interesting work being done by a group called "CommunityLab" in regards to how online communities sustain themselves and motivate community members to contribute.
Here is one recent study and the findings in regards to how feedback about amount of "effort" might effect a person's contributions. Harper, Chen and Konstan (2007) report on a study in which they used an email letter to community members about their relative number of ratings of movies on a movie-rating website. They divided the participants into two groups. The control group got an email letter that provided information about member's ratings in general and the experimental group was provided personalized information about how many movies they had rated compared to others-- less, the same or more. The groups were roughly matched in terms of past contributions.
Here are a couple of interesting findings:
1. Following the email newsletter both the comparison and experimental groups were active on the website. The authors don't tell us if both groups increased their activity from before, but this would be interesting to know.
2. Participants getting the social comparison data rated more movies than the control group. They rated twice as many movies.
3. Those individuals who were told they rated the fewest movies rated more movies following the feedback than any of the other groups. In short, the intervention worked.
4. There were some interesting differences between men and women that suggest that men felt more motivated to improve their scores when they were told were doing less well than others and women were more motivated when they were told they were doing the same as others. The authors did not seem to actually compare the actual behavior of men and women; they only compared their perceptions of what motivated them.
The author's make the following conclusion:
"While subjects who received an email message with the comparison manipulation were no more likely to click on one of the links or log into the system, they were more likely to rate movies. Thus, we find that a comparison makes no difference to a member's interest in using the system, bu that it change their focus within the system" (p. 157).