Short term, paying with FB for physical goods (eg. in restaurants, bars) is an unlikely outcome because carrier billing involves giving up an enormous percentage of the transaction to the carriers.
In the U.S., carrier billing typically involves yielding a 40 - 50% margin to the carriers. In Europe, the percentages are lower -- carrier billing is a more mature market there -- but they still top out around 20 - 30%. In developing markets, margins can go as high as 75 - 80%.
I don't know what sort of relationship FB has built with the carriers and what sort of percentage merchants using the billing system will ultimately end up with -- after all, both FB and the carriers will take a cut -- but I would be surprised if it isn't in line with the current percentages charged for carrier billing.
For virtual goods, carrier billing service providers claim the frictionless nature of the transaction and the reach (6 billion mobile subscribers globally) make up for the small margins.
Long term, of course, things could very well turn out differently.