Haha, I actually e-mailed Mark suggesting he cover this in his "is it primal?" blog post series. He said he'd try to cover it, but no guarantees.
When I was googling this, the only half-decent answer I found in an old MDA forum post here: http://www.marksdailyapple.com/forum/thread24422.html
Still no clear answer, other than it's super-processed and has artificial/lab-created B12 added to it, so it shouldn't be used in excess. I personally love the stuff and ate it constantly during my few months long vegan phase.