welcome to the community, we’re glad to have you here!
It is not Figma’s fault, but of the compression that’s happening when you upload to social medias.
When posting on most social medias, there’s a thing that happen when you upload a photo or a video: compression. Because of that, all the photos that are uploaded to the social media’s website will have a little worse quality (and be lighter too).
They do this to reduce the server load, save space, and to support slow connections too.
There is no practical workaround for this, as I know, because the compression is not made on your pc, but on the social media’s servers.
Also, I can give you a tip: when you design for specific social networks, look up for their standard asset sizes. For example, an Instagram standard post should be 1080x1080.
Figma comes in help showing some of the standards for different social networks (you can search them on the internet too):
It may not be the best explanation, but I tried .