Oricon compiled a sales ranking for the first half of 2015, revealing that 6 out of 10 top selling singles were 48-group songs.
Crazy, to say the last, especially considering that SKE's single, 12-gatsu no Kangaroo, was not even released in 2015. Notably, the top two sold over one million copies, with #1 close to 1.7 million copies.
2012: Manatsu no Sounds good!
It goes without saying that each single sold over a million copies each. Ameba news goes on to note that the only other music groups to achieve similar sales numbers are B'z and Mr. Children from the middle to late 90's, and even then both of those bands managed million sellers for four years in a row.
It did not mention online sales. I think Oricon can't track it however.