Zynga, the online game developer that made Facebook a safe haven for desk-bound farmers and wannabe Mafia hit men, has decided to cash in on all the fun with an initial public offering filing Friday that expects to raise $1 billion.

Zynga’s S-1 SEC filing begins with a chatty, casual appeal from founder and CEO Mark Pincus: “At Zynga, we feel a personal connection to our games through our friends and family. I love that my brother-in-law, who has five kids and no free time, religiously plays our game Words with Friends.”

But the filing also confirms that it is not all fun and games at the...