I mean, I don't know why Snowden bothered to leak the info, if they're just telling any 18-year-old German girl about it.
Assuming for the moment that this story did happen exactly as we're lead to believe (and I'm only making that assumption for the sake of playing devils advocate - I'm firmly on the fence with this story due to how sketchy the details are), then it would be very easy for the NSA to cover up how they had such data: "We'd been investigating her for a few weeks - here's a warrant"
Playing devils advocate here; there maybe more to this teenager than we're aware of. She might be an active protester or have left vocal remarks on other locations online that triggered closer inspection.
I'm not trying to say that your explanations are wrong or less plausible though, just that assumptions are being made on both sides of the argument.
I'm not saying that is what happened, there are other plausible explanations too. I am just saying that such a scenario is entirely plausible.
It doesn't seem likely a low level border agent would have access to this. If this is standard operating procedure, don't you think we would have heard about it from other people who were presented with printouts of their facebook messages when arriving at the border?
Or more likely, this is FUD that leaves out specific facts (border agents don't have broad access to facebook messages) to give the impression it is linked to a big trending news story.