Most internet-based forms of communication follow very different social formats than RL interactions in general - most of the same principles apply, but not all of them. I think in that case, a relevant difference is that people assume that a newcomer is a member of the social group until proven otherwise, instead of assuming that they're not a member of the group until they've made the appropriate social gestures.

It also seems like most people 'feel' anonymous online, even if they're using their RL names. The lack of body language is usually assumed to be the cause of that.