Union Square Ventures just announced an investment in the Zynga game network.

I appreciate Zynga for a number of reasons, but the most important reason is the simplicity of the investment thesis. Consumers love browser-based casual games. About this time last year, when you look at the top 20 websites on a “Time Spent” basis you find that Pogo (a casual gaming site) ranks #7, which is better than Amazon, Craigslist, and YouTube. So, casual games are huge.

But, one drawback of existing casual gaming networks is that they don’t make it easy to play with your real friends. Playing Chess against strangers on Yahoo is ok, but the conversation is usually limited to “gg” and “u2″ (translates to “good game” and “you too” for those inexperienced with acronyms in casual gaming). It’s not as fun as playing against your real friends where you can hold a side conversation as you play, or, if you’re competitive, you would actually care about the outcome of the game because there are real world social implications (aka bragging rights at stake).

Enter Zynga and social gaming. Zynga leverages the social graph you have already built in order to let you play games with your real friends online. No need to re-friend everyone or spam your friends with emails. If you and your friends are on any of the major social networks, you’re good to go.

Furthermore, Zynga alleviates the “now what?” factor that comes with most social networks. We’ve all experienced it… we sign up for a social network, add all our friends, and then wonder: “ok, I’m all setup… now what?” The answer to “now what?” is “go sink your friend’s battleship.”

More analysis from Fred at USV.com, and Brad Stone write a nice piece on Zynga in the NYT.