With SLComposeViewController, you can't pre-populate a Facebook post for the user. The methods exist in the social framework, but a few years ago Facebook disallowed apps to do this, so even if you use:
BOOL initialTextSet = [fbController setInitialText:@"Check out this article."];
the method will return YES, but the text will not actually get populated.
If you want to have more control over what you are posting, then you'll have to add the Facebook SDK to your project. I actually went that route in an earlier version of my app, but it becomes a pain to maintain. In the end, I just went with the Apple native framework. It's MUCH easier to use than the FB SDK, and it's future-proof. No need to wonder if your next iOS version build suddenly will have a broken Facebook share because the SDK needs to be updated (and it does, often).