Check out:
How to post on facebook wall using Facebook android SDK, without opening dialog box
and
How to post message on facebook wall using Facebook android SDK integrate android app
Here's a small example that will post to a friend's wall (or if userId is null then to the currently logged in user's wall):
protected void postToWall(String userId)
{
Bundle params = new Bundle();
params.putString("message", _messageInput.getText().toString());
params.putString("caption", "{*actor*} just posted a secret message.");
params.putString("description", "A secret message is waiting for you. Click the link to decode it.");
params.putString("name", "A Secret Message For You");
params.putString("picture", "http://www.kxminteractive.com/Content/images/app_logos/secretMessage.png");
params.putString("link", "http://www.kxminteractive.com/decrypt/" + _lookupKey);
asyncRunner.request(((userId == null) ? "me" : userId) + "/feed", params, "POST", new WallPostRequestListener());
}