You don't get spam on Snapchat? I think you might define it differently than I do. Since those walled gardens have a verification step before allowing sending messages, spam on those platforms consists of "friend requests" from unknown people. If one is foolish enough to accept one of those, the more traditional spam message soon follows.
So yes, most walled gardens' two-step approach makes the spam more civilized, but they don't eliminate it.
Not a single one! I suppose I occasionally see friend requests from unknown users, but they don't actually convey a message or achieve any possible advertising goal.
That said, I only communicate with a couple dozen close friends on snapchat so perhaps a more active user will get spam.
So yes, most walled gardens' two-step approach makes the spam more civilized, but they don't eliminate it.