The unit of sharing is the channel. You start with a personal channel user@domain.com and can create any number of new channels (holiday-photos@domain.com). Each channel has followers, moderator etc that you can control access to (or just set it as public): https://buddycloud.org/wiki/Channels_Explained

Think of the architecture like your email provider: You can always grab your mail spool and find a new provider. buddycloud is not dissimilar - just grab your posts and followers from the API and import them on a new provider. We're make this easier soon but, the data and endpoints are all there to facilitate it.

There's two three things that count when building a distributed social network: easy to use client APIs, Server APIs (how do servers find and connect, catch up, etc) and a reference implementation.

I'm not sure that Diaspora has a defined server-to-server API or a client API (can someone more aware of their progress comment). On buddycloud Each domain runs an HTTP API server (https://buddycloud.org/wiki/Buddycloud_HTTP_API) that clients can connect to on their home domain (eg if you are james@giantpeach.com, you connect to something like http-api.giantpeach.com). And servers use XMPP to exchange between one another.

Then there is Tentd. I like what they are doing. I think their approach is that every user is a server (your id is http://me.domain.com). I'm not quit sure how they federate or resync messages when nodes go down. Can anyone comment on how they federate?

I remember another effort in distributed social networking, from some months ago. Forgot the name though.

We know there are problems on the 1st gen iPad. Is that what you are using?