I feel like this is a wild guess, but...

<div itemid="https://www.example.com/link_to_current_page/" itemscope itemtype="https://schema.org/SocialMediaPosting">
 <div itemprop="sharedContent" itemscope itemtype="//schema.org/WebPage">
  <b itemprop="headline">
   <a itemprop="url" href="https://www.example.com/link_to_thread_1/">
    Thread 1
   </a>
  </b>
 </div>

 <div itemprop="sharedContent" itemscope itemtype="//schema.org/WebPage">
  <b itemprop="headline">
   <a itemprop="url" href="https://www.example.com/link_to_thread_2/">
    Thread 2
   </a>
  </b>
 </div>
</div>

I have no clue whether there HAS to be a container of itemprop="headline" or if I can combine 2 (eg, <a itemprop="headline" itemprop="url" ...>Thread 1</a>), but this came from the example from schema.org.

I feel like there should be other points that don't exist, too, like something to keep track of the count or page number? But the whole schema concept is nonsense, so who knows what they're wanting, really.

Taxonomically seen, SocialMediaPosting and WebPage are two different types of a CreativeWork but have not much in common. I would personally define it as a CollectionPage [schema.org], which is a subtype of WebPage.