In the past we've had verified incidents of people being refused entry because of things posted on for instance public Twitter feeds.
So we know that the monitoring of social media, collating that with the identity of incoming travelers and passing that on to officials at the border is an existing practice. We also know that once monitoring starts for one purpose, it rapidly expands to other use case. This pattern repeats itself around the world.
The only part open to debate is the suggestion of monitoring private Facebook communication.
So you're suggesting that everything published about the nature of US surveillance until this very day, including the recent PRISM-related publications, is all "utterly implausible"?
Because everything else I read in that article is perfectly consistent with what we already know as fact, and therefor completely plausible, albeit suspiciously thin on concrete facts.