POSSE is an abbreviation for Publish (on your) Own Site, Syndicate Elsewhere, the practice of posting content on your own site first, then publishing copies or sharing links to third parties (like social media silos) with original post links to provide viewers a path to directly interacting with your content.
I took that as POSSE is a principle that applies to more than just blogging. E.g. if you take photographs, you should not let an external photo sharing site be the sole place where your work is shared/stored.
I completely agree with that but I really can’t see where that differs from blog (or even the good old websites of back then)?
Sorry, it seems like we are talking past one another. I think the principle is that you have a site and use social media channels to drive traffic to your site and that applies if you are someone writing a blog or if you have other kinds of material that you want to post online.