When a post is made to Facebook that contain a url, the preview to that site loads. For example, if I post www.opera.com using my Desktop (latest opera) you will see a preview of the splash page. Not do with mobile. I can even go to the post on my Desktop and simply cut the url out of the post, and then paste (as text), and voila, there's the preview! If I post anything in the "what's on your mind" box and then paste the url in the comments, the preview works!
Phone is Samsung J7, opera version is 42.7.2246