Keeping mobile conversation private

Yesterday I witnessed an entirely silent conversation between two youngsters (students form the local college) on the bus.

They were sitting facing each other, both with feet up on the seats, taking it in turn to type busily into their phone. Then the phone was just passed over.

It was obvious from the reaction that some comment about another passenger or on something happening outside the window had been passed on.

I assume they were just drafting text messages, without incurring the expense of pressing send.