...who digests it? Who craps it out?

Don't say the guy under the suit....

In answer to the thread header, When Venom Eats Someone....he needs some serious fibre in his diet to keep him regular :-)

Thats a good question because Mac Gargan's stomach sure aint big enough to hold a human.
So the symbiote must play a signifcant part in the process.

the answer is that its a comic book. haha it doesnt have to make sense all the time

...Ouch...

Eddie Brock ate some dude's brains..
 
 
SOMEONE digested those brains..
 
 
SOMEONE had an interesting next morning, after they had had their coffee...
 
 
this is important!!

In my mind, being an overly-obsessive, way-too-old comic nerd that's had too much time to think about these things...
 
The Venom symbiote, whose mouth is obviously the one taking the bite, actually breaks-down the food it itself eats, and helps the human host digest it. This is the point of a symbiotic relationship; both parties benefit. Likewise, when Gargan or whoever eats through HIS mouth, the symbiote takes an equal amount of nutrition or whatever it needs to survive. 
 
When the Venom symbiote attaches, it attaches all the way through the body, I believe. It coats the inside of the host's digestive system, throat, etc. and more-or-less becomes a new stomach lining the same way it becomes a new layer of skin.  
 
I hope this helps at least give you a fairly reasonable reasonable answer to a qeustion I myself have had for years, now...