Facebook created a very specific version of their site for dumbphones a while ago, I think an article I saw about it said it was targeted at places like Africa etc where mobile phone use is becoming common, but computer usage isn't. Which makes perfect sense really, there's a much larger percentage of the worlds population that use phones over computers in less developed parts of the world. They're a growth segment, no point blocking them when you can hook them now.

Edit: http://www.readwriteweb.com/archives/free_mobile_facebook_wi... there we go, Facebook Zero.