It's best to
1) give users tasks to complete, such as 1: register 2: share it 3: etc
2) ask open ended questions like "how do you register?" and "how do you share it" etc... for the tasks you gave them.
3) score the responses: 100% means they did the task with no problem, 50% means they completed the task with some difficulty, and 0% means they didn't complete the task.
Low score = improve that area
Hope this helps!
We're dong the same with our new app, www.myboosters.com and it's working great.