I've never had a BD party given for myself, and have never attended one for a friend. The company I work for gives BD parties for people born in a certain month and I go because I know it would be considered rude not to.
I don't think it is endearing to snub other people just because they have different ideas, especially on something like this which can easily be accommodated. Does it really threaten G-d's superiority to tell someone else "Happy Birthday"?
I believe the Society's ban on certian activities is just a way to isolate their members. Granted I can see their arguments for such holidays as holloween and Christmas, however, birthdays are harmless. I personally do have birthday parties for myself, but we do have parties for my daughter. And no, I don't see anything wrong with saying "Happy Birthday" to someone.