I'm trying to composite a CG salt shaker on the table and have it look at least semi-real. With FG and AO it doesn't look too bad, the only problem is I can see the ground plane that has the Use Background shader on it.
It looks like
I've been searching around and I think maybe doing multiple render passes may fix it...but I'm fairly new to all of this so any help would be GREATLY appreciated.
Sarika's right about checking those settings... although usually the reflectivity of the Use Background shader makes it brighter, not darker, than it should.
What I suspect might be the issue is that the AO pass is slightly darkening the plane because the CG shaker is occluding it. Make sure that your 'max distance' for your AO node is set to a small value. If the shaker is 10 units wide, a max distance of 20 should be plenty high enough. This will prevent the shaker from occluding the most distant areas of the plane.
Not 100% sure that that's the issue here, but it's worth checking out.