The sad truth is that I know a LOT of teens who post that same stuff every day.
How they're so depressed and hopeless and so on, but these are all kids from seemingly good families and don't appear to have any issues other than their own attitude, so it's difficult to tell which ones are joking and which ones are not.
That said, the mention of a gun is a HUGE red flag and someone should have stepped up and done SOMETHING to try and reach her.