TLDR: it was because Disqus added the Facebook SDK in the last week or so, for some new feature they're testing. They're looking into this.

^ That sounds legit to me... I believe this was the primary reason why Facebook made an SDK and Like button in the first place...for data mining. Pretty clever.

This is the consequence of building on a platform like FB, you exchange your visitors browsing habit data for access and FB expands their graphs of IP<>websites to improve their ad targeting. And with Disqus is won't be as obvious because the publisher might not be aware that it leads to an FB connection.

So regardless if it was unintentional this is a relevant story for the trade offs of using platforms.