I don't think the rudeness is in posting it to Facebook, but in not being transparent about the kind of blog it is. If he'd said, "Hey, come see my ranty blog about politics!" or whatever, that'd be one thing. But usually when someone says, "Check out my blog," I assume it's a regular old personal blog.
And I think, even in your "private" corner of the Internet, you can still be rude. If I'm in my own home, and I insult people who come to visit, it's rude. I may have the right to be insulting and rude, but I'm still rude.