Dollars don't provide any what you call 'direct service' (if you don't use them as a wall paper that is), or 'use value' (in Marks terms I think), they only value is 'exchange value'.
Dollars, like all other fiat money, originated as a commodity backed currency, either with direct backing of a commodity, or indirect backing of a commodity by being backed by another currency that originated as a commodity backed currency. Bitcoin is not a commodity in itself (only directly serviceable goods can become a commodity), and has no direct or indirect commodity backing it.
It is possible that money started as commodity - but the point is that what differentiates money from commodity is that it has much higher 'exchange value' then 'use value'. This is really what defines money - so when you have something that has only exchange value and no use value - then it is kind of 'pure money'.