Medium might be a threat to WordPress, if there were no Facebook. But Facebook is there, and WordPress's customers are under a lot of pressure from Facebook, given that many of them are news publishers, and if WordPress wants to keep them, they need to evolve to help them survive the challenge, by keeping the independent and open web both independent and open.
Medium must also be a little freaked about the Facebook juggernaut. After all, what does Medium do that Facebook doesn't already do on a much larger scale?
Please go back to Hoder's excellent piece, which I think should be right up there with Barlow's Declaration, as one of the seminal pieces of the web, even though it was written just this year. It reminded me that what Facebook is offering is not the web and it isn't the blogosphere, although at first you might be tempted to say it is. (I did say that myself in a discussion on Rebooting the News a few years back, I was wrong.)
Both Medium and WordPress offer the ability to link from a word to another page. As Hoder reminds us this most basic of features of the web. I called this Holding Hands in Cyberspace in 1996. It is the central idea in my Rule of Links piece I wrote for the inaugural BloggerCon. Yes, Facebook provides links in the rarely-used Notes feature. But the typical Facebook post doesn't have them.
Also, both WordPress and Medium offer discovery, as does Facebook with its timeline, although Medium's is better developed. WordPress clearly understands that it needs to provide this function. I would like to see them lead their customers into producing their own rivers. I see this as essential to not letting Facebook run away with the whole thing.
WordPress and Medium are small competitors to Facebook, and both have an investment in the open web. I'd like to see them work together to strengthen the open system against dominance by the silo. I think it's really not a good idea for them to view each other as the enemy. Later, when and if the web survives the challenge, we can talk about them fighting each other for dominance.
This is an instance of the Prisoner's Dilemma which was so well described by Benjamin Franklin: "We must, indeed, all hang together or, most assuredly, we shall all hang separately." It seems Franklin understood the tech industry, even in the 18th century.