According to Unity 3.5 roadmap.
We are working on a pluggable social
API with backends for Facebook and
Apple Game Center.
I’d like to know if it’s possible to integrate facebook in current version Unity 3.5 by using built-in API (not 3rd party plugin). If it is then can someone suggest me where should I look at? I can’t find the doc about this in Unity 3.5
As far as I know, currently you can only access the documentation to Unity3.5 locally at "UNITY_INSTALL_DIR\Editor\Data\Documentation".
And from looking at it, currently, the only social API implemented is iOS Game Center, so, you will still have to use an external plugin to interface with Facebook.
By the way, if you are targeting WebPlayer, it is very easy to integrate Facebook support using only the WWW class.