I'm afraid you're screwed whatever you do. The data may be primarily stored on your own machine but you still want your friends to read it. This means if nothing else you have to at least send your status updates and photos to their boxen. They might be accessing a commercially hosted diaspora instance with IE4 on Windows 95A... and you can't help that. You've lost control of the data as soon as it got to their seed.
Compare that with Facebook. Again, they could be using any browser at all. They could have any of a huge number of viruses, keyloggers, webpage scanners running while they trawl through every photo you've posted, as you've granted them the right to do as your friend.
I think the competing model you're thinking of is the postal service.