Last week we wrote about Zynga surpassing the 200 million monthly active users milestone, but how many of those users are unique? Today the company is sharing that they have over 100 million unique users each month. While Zynga also has games on the iPhone, it would be safe to say that almost one-third of Facebook users are playing Zynga games each month. 66 million users have played Farmville in the past month, making up a significant portion of Zynga’s traffic.

Many developers are regularly critical of how we group monthly active users but don’t measure unique visitors. Being able to measure unique visitors would be extremely valuable, however we don’t have access to that type of data currently. Even with 100 million monthly unique visitors, Zynga is almost double the size of recently acquired Playfish. That shows how large of a force Zynga is.

The company even claims that 100 million monthly unique visitors makes them the largest online gaming destination on the web. While we don’t have numbers to verify this claim, we don’t have anything data to suggest that there is a larger gaming service on the web. With a rumored $200 to $250 million in annual revenue, Zynga appears to be monetizing their user base slightly better than Facebook who is estimated to generate over $500 million this year.

While it’s impossible to keep track of exact revenue numbers since Zynga is a private company, it’s clear that Zynga is experiencing phenomenal growth.