I believe they are saying keep the transactions the same (so 1x $10) but label the stored value as dollars instead of credits.

IIRC, most credit card fees are percentages, so the number of transactions wouldn't matter. I've never dealt directly with it, so I'm not 100% certain though.

Credit card fees are a base amount (which varies based in the auth method) plus a percentage of the sale. So for smaller transactions, the effective percentage is higher. Have you ever been to a store with a minimum purchase amount for credit cards? This is why.

Card fees depend on how big you are though. Large UK supermarkets have no minimum purchase amount for credit cards, because they have the clout to negotiate a better deal. Facebook is probably big enough to do the same.