If you use the Facebook app to post anything, it will show up "via mobile", but if you're a true iPhone fan, i bet you'll be annoyed by this thing, so here's the method to post whatever you want "via iOS"
1. Open Siri (by pressing and holding the Home button)
2. Tell her to "Post to Facebook"
3. She'll ask you what you want to post, just say anything simple, like "Test"
4. A small pop-up will be displayed with your post in it, just press anywhere inside it and edit the text as you wish
5. Use the Post button to send your update status via iOS to your Facebook Page