Is that really true? Why are people using Snapchat then, instead of SMS or even twitter?

It seems to me that the entire premise of Snapchat is that their users care about privacy (even if they are rather naive or uninformed about how privacy works).

in my opinion it's not popular because its private, its just easier to use to send pictures/videos to other people. the time limit makes it seem more "in the moment" good for a quick laugh and then its gone. a big plus is it lets you easily caption pictures and draw on them.