One Backend Per User: 100% data privacy / 0 scaling issues
Build apps, leave the back-end hosting to your users. Serve static HTML or mobile apps and let users connect to their own back-end, for example using Dropbox.js, the GoogleDrive JS API, NimbusBase, or the new open standard for per-user data hosting: remoteStorage.io.
This solves three hard problems at once:
1. Privacy: The users own their data and never send it to 3rd parties.
2. Scalability: as an app developer, when you don't own the back-end, it makes no difference if you have 100 or 1 million users.
3. User Data: since the user has only one personal back-end, common user data collections like the user's address book, calendar, tasks list, and bookmarks collection only exist once, and such data is already pre-filled when using a new app for the first time.