I now have around 20 amp pages that are correctly linked up with a canonical tag to my block posts. I have noticed that platforms that use amp such as Facebook & Twitter don't load the amp page for mobile users. Do I have to link to the actual amp URL to do this? If this is the case, that's not great for desktop users since the goal of this surely is to only serve the amp page to mobile/tablet users and maintain showing the original version to desktop users?
In the same way that you link your AMP pages using the canonical tag to the original document - you should also link your original doc to the AMP version. This will allow other servers to know where the AMP page is located - and it's up to them to show the AMP version or the original.
Here is the tag to point to the AMP version of a page from the original source.
<link rel="amphtml" href="http://www.example.com/amp/article"/>
The AMP page can then have the link back to the original.
<link rel="canonical" href="http://www.example.com/article">