Someone switching from FB to Instagram does not genuinely count as someone leaving Facebook; they are just switching to a different implementation of Facebook owned by the same company.
And the only reason most of them are switching is to follow the herd. They don't want to be left behind in some place where they are not able to obtain as much validation (fishing for likes and followers). That type of personality needs to be "where everyone else is".
Their lives are exactly the same, except with a different social networking application.