I searched for this before, but couldn’t find a solution that was easy or cheap (by cheap, I mean free).
Pavel Kultyshev (@getencapsulated) also wrote a blog post, sharing messages on Twitter or Facebook by following the link in the browser of a mobile device or by opening the client app on this device. I haven’t tried this approach though, because I’m looking for something more “native” to Unity.
If you’d like an easy drag and drop solution and willing to spend, then there are a couple of available plugins: the aptly named Facebook Plugin for Android by Mida Mobile (for $20), Unishare, that also includes Twitter, LinkedIn, Sina Weibo, etc. (it’s $25, I don’t know how good it is, but for 5 more dollars you get way more social stuff, sounds pretty good), and of course, Prime31 also has one, it includes Twitter too (for $65 though).
Macaronics (phardera) also wrote a blog post on how to do this using Android Plugins: http://phardera.blogspot.tw/2013/04/unity-3d-facebook-sdk-for-android.html (it’s in Chinese though). Don’t worry, the entire project is available in phardera’s github (https://github.com/phardera/unity3d_fb_android).