I like the concept a lot...enough to worry that I would waste too much time on it :)

Putting it on Facebook is a smart way to reach a wide audience, but (like a lot of people) I have lately become allergic to the idea of Facebook apps due to privacy concerns (with FB, not you!). So I urge you not to limit yourself to that platform - make it accessible to Twitter and Google users too, who will also bring you plenty of traffic.

A smaller grid might be needed for mobile phone users - I hope you'll consider an Android client. I could definitely enjoy a casual multiplayer game like this at the bus stop or during lunch.