I thought the weekend effect referred to the fact that Saturday and Sunday generally have the heaviest traffic on iTunes. So a song that is only T20 on Saturday may actually sell more copies than a song that's T10 on Wednesday.
The ratings for the XF UK in the US are infinitesimal.
However, the video of Matt singing his winner's single was trending on YT. That's a more likely source of American interest.
Louis needs other people to tweet. It's one thing for him to appear on XF and dedicate the song to his dead mother. To actually get on Twitter to promote it would be much too crass.