still can't make a post shareable on facebook - what am i missing?

If it's already set to Public and the button still isn't there, check whether you're looking at it inside a group. Group posts follow the group's privacy, not yours. You can always copy the link from the three dots menu and paste it anywhere, but it will only open for people who are allowed to see the post anyway.

the share button only appears on public posts. friends-only ones can't be shared by anybody, and that's usually what's happening. three dots on the post, edit audience, set it to public and the option turns up. there's a separate setting for who can see your future posts if you want that as the default. the one thing you can't get round is a post inside a private group — those never share out.