It ranges from 10% to 40% or so. WHY do they do this? Because fb wants to make money and if you want to guarantee the % who see your post you need to pay them.
How do they decide what % of your fans see your posts? By how many replies you get to the posts you have on your fb page. I was VERY happy to see (you can look in the admin section to see what % of walls your posts were put on) we are at approx 35%. Reason is(dumb luck on my part) that when we make a post we almost always are running a contest and require people to SHARE it to be entered in the contest.
Sooooooooooo if you are posting lots on interesting facts, info etc on your fb page you are killing your % of people who actually see your posts because very few if any people will click like or share on these sorts of posts.
I truly think that fb intentionally did this so the number of your fans who see your posts will go down down down and you will need to pay fb to change that.
The same way that Google changes their SEO 'rules' so that you will become frustrated and pay them for clicks