On that particular aspect: all Facebook employees (and I believe interns) when they write on Messenger to other employees have an on-screen reminder that their conversations are considered work product and can be monitored or reviewed. At least, that was the case when I was there, I can’t think it has changed. I’m assuming this has to do with a lot of well documented legal reasons (insider trading, sexual harassment, etc.) De facto, hardly any conversation goes through email: salary, pension, that kind of things almost exclusively. Project progress, being cancelled, blockers, all that goes through an internal version of Facebook (now Workplace, when I was there, a subset of the main Facebook). It made sense that all that is deemed work product for the company.

I would be more surprised if they had access to conversation with outsiders.

On the “genuine concern”, I joined several months after Snowden’s revelations. The security team was (and I’m assuming still is) very aware that State agents might try to access employee credentials. Overall, the company is very considerate for employees, typically no matter the cost (e.g. the freezing eggs things) and quite conscious to respect boundaries--but I would expect that a small chance that a hostile Nation state escalated to kidnapping employees is a stretch but not a big one. I’m a little surprised because the company prides itself on not judging, and actually doesn’t care much about time spent in the office. Interns might be given a shorter lead there.

With E2E encryption introduced to Messenger since a lot of that could have changed.