While making Plone an OpenSocial container might not be that useful
right now it might nevertheless be useful to look at what they propose
regarding a RESTful API:
http://docs.google.com/View?docid=dc43mmng_2g6k9qzfb
This might be interesting because even without rest of OpenSocial it
is an example of how social graph information can be exposed and maybe
even edited.
It's a proposal though and it needs to be seen how much it gets
adopted.
-- Christian