Dropbox has a brilliant way of engaging users in their application and getting them to share with friends. They even committed an entire tab to it.
If you complete 5 of the 6 tasks, you can get a reward – which is more storage. Note, your reward is their reward. More storage means more engagement.
Regardless, take a look at the requirements and you can understand their priorities: learning (how does the app work), permanence (downloading it to your desktop) and sharing (social marketing).
