I read it more as Wordpress does Diaspora better than Diaspora than it as a serious competitor (without the open qualifier) to Facebook.
But there is a strong part of me that thinks WordPress could become a serious (although very different) competitor to Facebook.
I'm not sure that Wordpress will ever be a real competitor or replacement for Facebook, although Wordpress is pretty popular as it is today.
Whatever "the next Facebook" turns out to be, it is unlikely to look anything much like Facebook! It will need to do one or two features well that Facebook does badly or not at all, and be a means to a particular end. For example, the original focus of Facebook was to share contact details and let people communicate about events within a relatively small group of people (students in specific universities). The interesting stuff happened elsewhere; the site merely allowed it to happen more easily. I don't think that an open/distributed clone of present-day Facebook is going to be enough, which rules out Diaspora and friends.