If you want to spread by word-of-mouth, you do need to give your users something worth telling about to their friends. Any constraint is ok. Look at Instagram (

If you want to spread by word-of-mouth, you do need to give your users something worth telling about to their friends. Any constraint is ok. Look at Instagram (filters) or Path (limited number of friends). From my perspective, you are a DailyBooth which costs money. Why not tap into all the photo apps already out there? Hook into their APIs and automatically generate albums for people. This is a great time to sell "My life in 2010" albums. Once you get people to pay you money for something, you can ask them again and they will gladly pay for other stuff too.