> I'm assuming Facebook, Twitter and Myspace are not paying Bell Mobility
It is quite likely that they are, see the nice error message you get from http://0.facebook.com . I suppose the deal requires Facebook to pay ISPs; the ISPs are free to offer free access to any site whenever they want and I doubt they are really after a text-only interface.
But this is not the point. The main problem is that all the Facebook competitors will have to pay if they do not want to be relegated in the I-have-to-pay-for-it separate internet.