This is what we're building with Tent (https://tent.io). The idea is since you can always host your own Tent server, it doesn't matter if your hosting provider shuts down.

You can migrate your data to another server and change your entity (global user identifier) as part of the protocol, so there's no messy "please update your address books" (that part happens automatically.

Data is stored as posts and post types are developer extensible (posts are JSON and binary attachments), so if your favorite app is discontinued, another developer can make a great app that interacts with the same post types.

We're working on a Tent app that imports Posterous posts right now.