At this point Facebook is so big that "millions leaving" could be people dying, people losing their accounts, etc. It's very small compared to their scale of more than 2 billions users.
That’s true, if that’s where it ends and doesn’t represent a burgeoning trend, and if it doesn’t have a feedback effect leading to greater losses. After all if a fire starts in a mansion, well the fire is tiny and the mansion is huge, why get all excited over nothing? You have to leverage what you know about fire if you want to appreciate the threat it represents to the structure.