If you have a billion users and you charge .1% of them $1,000 that's $1 billion. If you charge another 1% of them $100 that's another $1 billion. If you charge another 10% of them $10 that's another $1 billion. That's $3 billion already and we're just playing with rough numbers.
Text messaging has certainly been driven by user fees. If you look at the tv market it's moved from primarily ad driven to primarily fee driven.