iOS has sharing to Facebook, Sina, and Twitter built in now. Look at Social.framework, it’s really easy to implement. As for Android, I think you can just use the action_send intent. You’ll want something like this:
Intent sharingIntent = new Intent(Intent.ACTION_SEND);
sharingIntent.putExtra(android.content.Intent.EXTRA_TEXT, "My message to share");
myActivity.startActivity(Intent.createChooser(sharingIntent, "Share using"));
That’ll pop up a little box and allow the user to choose a sharing app to share with.
I hope that helps!