you're running in to the facebook bug. more information on that is discussed as "the second thing" in this post:
https://www.ubbcentral.com/forums/ubbthreads.php/topics/260384/re-mobile-view#Post260384

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The second one is a bug in 760 that has been fixed in 761. its related to the Facebook images on the footer of each post.

Basically, that element under each of your post threads which says, "Tweet". There is a Facebook element right next to it that has a 450px wide iframe, which is preventing your pages from being mobile. This was a very old style of presenting Facebook's like system. They have since updated it to use javascript. There should be a Facebook "Like" phrase right next to Twitter's "Tweet" text also, but I am not seeing it displayed on your website on any of my browsers. So Facebook may have stopped using that old format all together (?). Though the 450px wide iFrame still exists in 7.5.x and 7.6.0's template.

backstory: while working on UBB.threads 7.6.0, I was unaware that there was even Facebook or Twitter integration at all. So it was never noticed until someone mentioned it about two months after the 7.6.0 release. This has been fixed for UBB.threads 7.6.1 by updating both Facebook and Twitter share buttons to follow their currently recommended integration code. In addition, a Facebook "Share" button was also added to 761.
The 760 bug was documented and fixed for 7.6.1 @ http://www.ubbcentral.com/forums/ubbthreads.php/topics/260038#Post260038

The fix for this Facebook iFrame is to just disable that integration in your Control Panel until you get 7.6.1+. This will remove the display problem. This does not affect Twitter at all. You can continue using the 760 Twitter integration without issue.

1) go to Control Panel > Display Settings
2) And then open the "Topic & Post Page Settings" section.
3) Remove the check-mark from the "Show Facebook "Like" Button" box.

Last edited by isaac; 11/06/17 11:11 AM. Reason: added relevant quote