I totally agree with what you are getting at, and maybe “portability” is the right way to think about the next step. But, I personally have trouble seeing these social networking sites as actually embodying my social networks, and I react to their offerings as being mismatched to my needs.

I actually think that these social networking sites are successful in part because of the novelty of finding one’s social network in a closed and constrained system. In other words, the mismatch in general and the lack of portability / openness specifically are core novelties / features of these sites.

So, I wonder if the “portable people aggregators” are just very different applications altogether.