I understand that terrorists tend to be pretty stupid, but I'm not sure even they are dumb enough to think Facebook's encryption is that secure. My understanding is that Third party e-mail/message encryption is essentially unbreakable in any useful timespan.
Device encryption I'm fuzzier on. Obviously Apple's wasn't as good as they thought, but both they and Android supposedly upgraded it. If I had stuff I cared about, I'm not sure I'd trust the big guys.