My friend who interned for them last summer referred me to a Zynga employee because of my iPhone development skills. A person from HR promptly contacted me and asked to do a phone screening. In 2 weeks I talked to an HR who asked basic questions and simple knowledge questions like "Are you familiar with vectors, what are the most important concepts in programming, etc".
He then said he would get back to me. 1-2 months later I sent a follow up email (you probably shouldn't wait that long) and he was happy to move me along the interview process (it seems he forgot. and after speaking with my friend who interned there as well, this is common. Being a startup I assume things get pretty hectic so you should stay on them). I was then scheduled 3 phone tech interviews for the following week. The questions mainly involved PHP/MySQL questions, and some related to multi-threaded programming. The intervieweres were extremely nice and were willing to give you 'hints'
I got a call in 3 business days from HR saying that I got the internship.