Only Girl is a great song. Just because it didn't perform in the US as well as it did worldwide, doesn't mean it shouldn't been the first single. It just has more Worldwide appeal than the US, since we all know how diverse the US market is.
And if WMN can hold it lead till Sunday, it'll be Rihanna's 9th digital #1. That's good, even if it doesn't go #1 on BH100.