I was about to dismiss this but have thought about it a bit more, and there's something interesting here. There are already APIs built on top of you. There are the ones you know about, like Facebook and Twitter. There are the ones we know exist but have no direct access to, such as Amazon's backend or your police record. And there are the ones we may never know about, such as the NSA bot scanning this post right now…
Social software is very complicated because of issues like this. Something like a personal dropbox is straightforward - single customer, no community. You only have to worry about people hacking you, not what they do to each other. Anyone who gets into social software inevitably sets themselves up for making rules and judging disputes, and that often requires hard decisions.
Or compare email with social messaging; the rights and responsibilities are different and it's due to the software. Personal servers only support setting ground rules in a certain way (for example, the inability to revoke or prevent copies). Centralized services can support communities with different rules, for better or worse.