I can't try this because there's no way to use it without a facebook account.
What's a "posterwall?"
A wall with posters. Well, a virtual wall with posters.
Is this a Thing? Should I already know the use case for a posterwall? Do people use them as backgrounds on their Twitter profiles or something else that I'm too old or out of touch to know about?
And to be sure: I did assume this was for actual walls.
Yes, you're actually right. I'll try to come up with a new description.