The s1 listed 50m shares as coming from existing share holders but they did not break it out. Usually that is employees with vested shares and occasionally other investors. Was more common in the dot com IPO's but Zynga did that too as I recall
The bankers want to strike a balance between employees (especially officers) who have concerns about being whipsawed with the stock price and the perception that they feel the company will grow in value over time so holding the stock is a "good" investment.