The39thStep
Urban critical thinker
I've only met a couple but from what I can tell it's as more about having a good time than it is about principles (which is probably a better reason anyway IMO). Get in cheap, have a laugh with your mates etc. I think most of them still describe themselves as Man U fans as well as FC Unite fans.
Their funding plan for the new ground is a really good example of how to involve supporters without just relying in donations and goodwill.