Luke Wroblewski has some good advice on getting users to do what you want without driving them nuts. In any successful app, users develop muscle memory around common tasks. This means it’s difficult to push users to complete secondary tasks — like adding a user photo or connecting with friends — that will improve their experience but are rarely the reason you open the app.
His solution is to style these tasks like other items in a user’s feed and insert them seamlessly. He didn’t give numbers, but said the “use of the Find Friends feature shot up dramatically” after implementing this change.
It’s a simple observation, but a powerful one for me. Changing someone’s behavior is difficult and there’s no reason to take on this task when it’s unnecessary. I would just be careful to respect the user’s intentions and avoid polluting a stream with too much unexpected content.