I signed into Facebook Mobile last week. It's very convenient to receive and send updates for the Facebook mobile. However, the service is kinda of expensive since my AT&T carriers will not only charge me messages I send, but also messages I receive. I would call the second charging "passive payment", since I couldn't really control on how many updates I will receive from facebook every day. It's a business model which expels customers from using the SMS service more.
That's the situation in the fragmented U.S. mobile market. Since there are many carriers, the interconnection cost is so high that customers have suffered a lot of benefit. In the terms of MicroEconomics(which Amy is studying on), the high price of interconnection(such as SMS between different carriers) have created a dead weight loss in the whole revenue map. Both carriers and mobile users leave money on the table.
Internet could help change all this. There was once a website which has the potential to challenge the whole situation. It is the skype. However, since Ebay itself doesn't have a lot of operating experience in telecom, Skype now becomes something stuck in the middle without any future innovative move.
For Facebook Mobile, I would suggest them learn from Tencent in China, which provide the service of Mobile QQ to customers. (5 yuan per month for unlimited messages between online and mobile chatting service, that's the price I remember). It is the only way to attract users to the SMS service, since we know customers always want to control on how much they gonna to pay.