Yes okay, happy birthday whoever you are.
I don't do much. If forced by friends/family, I will usually end up getting drunk/getting drunk and embarrassing people. But I would be quite content to let the day pass without any special attention. I consider it a day like any other, except for the fact that 21 years previous my mother was in a world of pain and swearing at doctors.
As for making mention of my birthday on forums, I get enough unwanted attention from people who actually know me, I don't need more attention from people who don't.