Say Facebook charged US $1/month/user.

$1 is more for someone living in Argentina than in the US, and less for someone living in Europe. That seems bad -- you want to get people to pay what they're willing, and no less (price discrimination). Europeans would pay more, Argentines less, in USD.

Now, say you charge local prices. What's to stop someone from the US changing their network to Zimbabwe or whatever?

Once you start charging you start distorting how your users behave. Since Facebook rests on network effects any small change in user behavior has a huge impact across the network.

I think getting users to pay Facebook directly makes sense, but they have to be careful about it.