If you have added or changed a thumbnail image or changed the name of your paper, the number of days for the changes to appear will depend on the platform.
For Facebook and LinkedIn these changes can take up to seven days.
This has to do with how the platforms handle their data and how often they clear their cache and is unfortunately nothing we can control on our side.
The Facebook development team has created this nifty tool which allows you to clear the cache of a URL yourself.
Paste your paper url in the field and click "Debug". Then, to force Facebook to pull the new image or title, click "Scrape again". Your image should appear when you refresh your browser page.