Walled gardens
There are quite a few interesting articles out there which look at the problems arising from having “walled garden” social networks. ZDNet has an interesting article which looks at the “social capital” people have locked up on different sites such as Facebook and LinkedIn. Kottke talks about Facebook specifically, comparing the closed network with the previously walled-off AOL. In any case, it seems to me that these networks will surely have to open up in the end as people get frustrated having their contacts and other data duplicated and walled-in on many different, incompatible networks.
ZDNet article here, via Twitter, here. Kottke article here.
Further reading
These posts may be related to the one above:

