Native German speaker here. The article alleges that "apparently" authorities had been monitoring their private correspondence for weeks. I highly doubt that this was the case. Usually it works like this: Once you're deemed suspicious at the passport control, you're sent for "secondary inspection" to an office in the back. There, you're being questioned further, and officials will search publicly available information for anything suspicious. This includes your Amazon wishlist [1] and in some cases searches of personal items like laptop data [2]. The most common case is when future au-pairs bring physical letters from their hosts with them, and CBO agents find them. There have been thousands of stories like this going back for decades, that's nothing new. What's new is the ubiquity of social media.

In this case, I assume the girl was stupid enough to post publicly on the wall of her future employer, and the CBO agents could easily find it and print it out. I seriously doubt that their private conversations have been monitored. That just wouldn't make any sense.

[1] http://www.heise.de/tp/artikel/16/16039/1.html [2] http://en.wikipedia.org/wiki/Border_search_exception