I also find it ends up looking rather spammy. A blog article is written, and then it's posted everywhere in an attempt to drive traffic to it. It's often hard to see a difference between someone practicing POSSE and someone spamming in an attempt to help their SEO. This is especially true of 100% of their posts are just links off to the blog, where they treat all the social platforms like alternate RSS protocols.

A social networking site designed around POSSE may be different, where you can subscribed to your blog as a means to post, and the post shows up as the RSS would in a feed reader. This way people don't have to click through to read what was posted, or can at least read what is above the fold. This can be rounded out with comments, one-off posting, and maybe some standard way to write a blog post that references another, for a proper linked/threaded response for more thought out and thoughtful replies than a short comment.

I fully accept that my view may be dated to the point of having inverse consequences (maybe in line with what you're saying). But, there's just no getting around the feeling I get when I see the exact same post, in the exact same context, showing up on every platform I use. There's just no way that can't feel like spam. And when I do it, it feels like I'm spamming people, too. Having come up in the blogging days of 2003 on, I'm just sort of programmed that way now. But, like I said above, I get why people do it.

Side note: It's such a bizarre thing that the platform you're on matters at all. Not without reason (they all have a vibe now, that's basically politically informed). But, back then, you were just on whatever blog platform was the easiest. The platform was more or less invisible (or at least ignored).