Hi,
I am currently not set up to provide help on Android unfortunatly. I have IOS, Windows and Blackberry ( received my device today actually
).
I do plan to get an Android device soon ( thanks Santa...) as this would only be a testing device, it's always a bit of a bummer to invest so much in a device you won't actually use much... oh well...
So, I am not aware of any Android specific custom actions on that front, However, Facebook is not integrated in Unity very well, have you studied that aspect already? what is covered on Android front, I am not sure but Untiy and Facebook are working very closely now.
Also, the Asset Store contains a lot of generic social solution, have you looked in the asset store as well?
Bye,
jean