"Porting the data" isn't the problem. Porting your friends is. Nobody is on Facebook because they have all their data on there - they are there because their friends are there. The issue of writing the software, even the issue of data export, pales to total insignificance next to the problem of exporting the network effects.

I didn't say that many open source implementations exists, just that many implementations exists. Diaspora might be a good candidate for a software stack, but the point is that having some social networking software is utterly pointless if you don't have a way of getting users onto it.