After looking for a while I find no real solution to my problem...
I rendered a video in adobe premiere CS6 to upload in youtube and facebook with the following setting:
Frame Rate: 23.976
Bitrate Encoding: VBR, 2 Pass
Mbps: 120-140 (Just so there can be no excuse)
Selected "Render at Maximum Depth"
selected "Use Maximum Render Quality"
Now both youtube and facebook show some crazy fat pixels when shown in HD specially from 0:42 in the video.
What I find very strange is that those pixels are not seen when you watch the video in lower quality.
I know the background is complex with lots of trees and that is not ideal but as you can see on another video I made it is not near as bad as the new one...
Here is the last video showing extreme pixels on some transitions from 0:42 to end:
Here is a previous video without those huge pixels and the background is still pretty much as "complex":