My suggestion - make a dedicated page to all contests past, current, and future - with the winners posted there as well. (not saying take away facebook/instagram etc - they can be used in conjunction).
But having one place that can easily home the information would be great to streamline searching for who won a contest.
Idea / example zwift.com/contests … each contest is listed - can be clicked for more details and has a dedicated area for the winners - heck this could even work as a plug for them to post something about winners (pictures, joys of winning etc), event stats - like how many participants would be great to know/ interesting too.
This would also be great to know what contests are going on since many of them you have to click into the individual event page to find out if there is any sort of contest associated with it. I would imagine this would help promote engagement as well.
Pretty sure other places have a more streamlined method for contests - Rouvy as an example.