SN-API-capabilities-comparison is a project mainly written in ..., it's free.
Common Social Network API capabilities comarison table
This document in MultiMarkdown (easy readable in plain text) is a table that tries to maintain a comprhensive table that show what Social Network's API are able to do or not.
The terms of the capabilities are general so the term could be significative for many Social Network not just one (ex. Retweet makes senso just for twitter)
This could be useful for the outgrowing social aggregator out there in the interwebz to know what can/cannot be done
Any help is HIGHLY appreciated, foremost by whom knows a particular API better than me :).