"Number of users per day" is a vanity metric. It doesn't matter to a business like Yahoo. The thing that actually matters is whether or not those particular users make money for Yahoo. If they're run the numbers and discovered that Facebook users don't turn in to paying subscribers or click on adverts (and they can't find a way to do that) yet cost them $0.25 per day in bandwidth then turning off Facebook login will increase their profits by £200k/month due to the saving.