"reading" doesn't necessarily have to be done by a human. As PRISM has shown, the government has capabilities we weren't aware (or at least sure) of. Would it be that ridiculous that they would have algorithmic scrapers running through all the FB data they have available? They say they aren't, but they've already proven we can't trust them farther than we can throw them. Even if they aren't, she's a foreign national, who they have almost no restrictions on investigating, so they may have been "reading" her messages, even if those of US citizens were left intact (which I personally doubt).
Long story short, it wouldn't necessarily waste much time to go through her records, and it would probably be completely legal.
The part that seems unlikely to me is that they would show their hand by using PRISM for such a small thing, unless there is some bigger reason they want her out of the country. It's hardly a sure argument, though.
Even if there is a bigger reason for wanting her out, there is no reason for them to expose PRISM that way. Immigration officers already have rights to deny anyone without explanation.
I just don't find anything about that article logical.
This particular case, if true, may have nothing to do with PRISM. A simple search of communications between the person and only her US contacts would probably result in a small amount of data to search, which could probably be obtained by DHS or CBP via the providers directly.
I disagree, It's already well documented how the US government follows social networks and decline people entry over comments made[1].
Where this case differs is that the comments were over private messages - which means that the intel would have been collated covertly. So the real question is whether this is another piece of the PRISM jigsaw or just a sensationalist article cashing in on the hysteria that the NSA leaks have generated.
Long story short, it wouldn't necessarily waste much time to go through her records, and it would probably be completely legal.
The part that seems unlikely to me is that they would show their hand by using PRISM for such a small thing, unless there is some bigger reason they want her out of the country. It's hardly a sure argument, though.