I believe on Facebook, 90% of my friends only post something when they thank their friends for birthday greetings. They mainly use it for Messenger, and rather use Snapchat with a much more limited subset of friends for sharing. Most either don't have more SoMe accounts, or they use Instagram like they do Facebook, to follow people.
I couldn't find statistics, perhaps mainly because googling "facebook post frequency" or similar gives you 1000 SoMe SEO tips&tricks pages.