They are facebook's largest advertiser, IIRC. Additionally, I believe facebook has invested in zynga, so the relationship is more symbiotic than parasitic. I am anti-zynga, but its important to be accurate in our critiques.
This doesnt mean they dont have their own infrastructure - but it is different than an MMO company like SWG/WOW that have their own infrastructure for 100% of the game.
The point made was that Zynga is doing what MMOs do at far lower cost. I said the cost was offset because Facebook foots the bill for the platform on which Zynga depends.
I believe Zynga provides more of Facebook's profits than any other company.
Zynga could easily argue that Facebook is a parasite by profiting from all the people who want to play Zynga's games.
Also, zynga is an idea thieving bandit of a company - so their costs are also far lower due to their hiring people that can copy as opposed to the thought leaders who would have come up with original content.
But if the only metric you choose to measure them on is revenue to expense, then by that myopic unethical lens - sure they look good.