The decreased activity in response to faces seemed to occur in those who achieved literacy during childhood (it was one of the only differences between them and adult learners). The authors suggest that the area that responds to faces normally expands with age, and learning to read may limit this expansion by putting nearby brain areas to other uses.According to my mom, I taught myself to read at a very very early age (she says that at 18 months I got obsessed with letterforms and reading the cereal box and so on, and I certainly remember being able to read at the age of 3 and wondering why all the other kids needed the teacher to read storytime books out loud), and I am also profoundly bad at facial recognition. So, while an anecdote is no substitute for data, it's certainly something to think about.
This isn't to say that you're going to be worse with faces if you know how to read well, although the authors indicate they're going to look into that to find out. But it does indicate that literacy involves a new specialization in some areas of the visual system, which ensure that the centers involved with processing language become just as active as if they had heard the words spoken.
Frankly, I feel that it's been a reasonable tradeoff for me.