> You control the source code. You can easily export your site from WordPress.com and take your site with you.
Only if you download WordPress from wordpress.org and install it on your own server (or shared hosting account). The dot-com version contains a lot of proprietary code.
> WordPress.com has already become a login option on many different websites.
Only if you opt to host your blog with them. WordPress.com is a great service, but hosting your blog there is mutually incompatible with having complete control over the source code. Besides, half the WordPress sites out there are self-hosted, and WordPress.com logins don't work there.
Some of the features the article mentions, like JetPack, may be available for both WordPress.com and self-hosted WordPress blogs. But that's just comments. There is no meaningful aggregation between WordPress.com and self-hosted WordPress blogs when it comes to posts and pictures. Until there's an easy way to follow a lot of WordPress blogs no matter where they're hosted (hello, RSS), it will be difficult to make the argument that WordPress the software makes a promising social networking platform.
What we really need if we want a distributed and open-source social networking platform is improved interaction between independent sites. Not with newfangled stacks like Diaspora, but something that can be slapped onto any existing blog, microblog, or photo sharing service as a plugin (again, think RSS but much more powerful). Why limit your network to WordPress.com users, or even all WordPress users? Give me a common API that lets me follow folks on WordPress.com, DeviantArt, and Pinterest at the same time, a gem that I can add to my Rails-based blog to support this API, and a nice open-source client that ordinary people can use to access all that data. If a few companies like Mozilla and WordPress.com started collaborating on this, it would be a thousand times more viable than Diaspora can ever be.