It's common for US immigration and border control agents (and Canadian agents) to do Google and other internet searches. This has been disclosed and acknowledge

It's common for US immigration and border control agents (and Canadian agents) to do Google and other internet searches. This has been disclosed and acknowledged in the US media several times. While the messages were not likely Facebook "chat" messages, they could have been "friends only" posts (or even public posts that the woman considered "private" from a foreign government). My understanding is that US law enforcement has access to Facebook "God view" where they can see all your Facebook posts. They don't need a warrant because Facebook owns the data and is the one giving them permission to view it. It's a different class of data than a "chat/email" in that it's posted, not directly communicated, so there's no reasonable legal expectation of "privacy". As for personal experience with this, I had a musician friend that posted on his website that he was going to be performing a series of shows in Canada. He was asked at the border if he planned to work while in Canada. He said no and did not have a visa that allows it. The border agent showed him his website, which was found with google, showing that he was planning to work. He was denied entry and banned from Canada for 1 year for lying to a border agent.