They are functionally the same when you are talking about online communications. If your message is not secret it is not private, the operator of the messaging service can see the contents of the message.
Private, in the sense of “contains sensitive information that I don’t want public” is just a feeling. Saying those things in a Snapchat “private” message is no different than posting them in a “private” message board. The number of people who can read those messages is reduced but still contains significantly more than you and your intended recipient.
I have a 2010 Mazda Pickup. It doesn’t even share with me.