I've been witness to the social loafing phenomenon
this very day.
I teach some computer applications, and the folks in the class were broken into groups. Right away, I noted some folks shifting almost audibly into neutral as the "go-getters" took the helm. It's very easy to loaf when you have someone willing to do the heavy work.
To combat social loafing, I added some guidelines to the projects. I made the loafers the team leaders. They have to know what is going on with each team member. When their project is designed on paper, I will ask questions of each team member to make sure they all know what they decided on. They can have points chipped away from the scores if they are not working together as a team.
After the new rules were in place, I noticed a reversal in the loafers. They are the ones driving the new innovations, which the go-getters are working on to develop. Like electricity, most folks will take the path of least resistance. Make the resistance uniform and fair, however, and you can motivate those loafers. I challenged another class run by another teacher to a competition to who designs the best project, with the Academic Dean as the judge. Those loafers are now so motivated, they're taking work home and planning field trips this weekend to collect data for the project.