I think it is a nice bit of camaraderie, a "human touch" that makes the board just that bit more of a group of friends. I try to post wishes for the people I've had more interaction with, or the people I feel contribute a lot here, or at least where I recognize their names!
That said, I know I sometimes miss some Happy Birthday threads where I probably should have/would have/wanted to post a wish, and sometimes I'm in a hurry and think I'll come back and post later, but forget to come back. I just hope anyone I DON'T post a wish for, doesn't feel "neglected" if I don't post. With that in mind, a HAPPY BELATED BIRTHDAY to anyone whose birthday thread I missed!
There eventually will come a point where, with only 365 days in the year and many thousands of members, one will never be able to keep up with them all. But since they aren't automatically generated, but someone has to check birthdates and decide to post the first birthday wish, perhaps it will always be limited to those who have at least one friend here who cares enough to post.
And even though one may wonder if a few people are just PW-ing, it is still touching to read so many wishes from people here, when your own birthday rolls around. You know that at least many/most of them are genuine wishes, because no one makes a person post in such a thread, and when you take time to do so, it is sincerely meant, too.