On a worrying note, I think this is the first time OP sales dropped considering the last year (38 million in 2011 vs 23.4 million in 2012).
Nothing to worry about. OP got a huge boost from the MF war to the end of part 1 with new fans coming in and buying old volumes. Right now it basically normalized, we should see numbers in the high 30s-low 40s down the road once some of the big mysteries start being unveiled.
As for Toriko, In my opinion the extreme focus on training, battles and power ups spanning the last 3-4 arcs might have contributed to it slipping in the rankings.