I always figured they'd go and do a payment system because in facebook the 'graph' of your friends validates you to the point that doing business with someone becomes a lot less risky than on paypal, but for some reason they seem to be holding back on that.
Maybe the zynga deal is a prelude to that.
It would make good sense to do that, facebook is the closest we've got to a verified ID on the net. The issues to contend with would then of course be hacked accounts and such but that's not different from other payment systems.