"Happy Birthday" or "Happy birthday"?

Which do you prefer, when writing on a greeting card or FB wall?

I do neither of those things, but I loathe excessive capitalization.

The first one

“Happy Birthday” is a song. I use the latter.

For Facebook I use, “Happy, happy, happy, happy!” or some variant. They know it’s their birthday.

I’ll write “Happy Birthday.” I can’t believe anybody is actually bothered by it or would be ill-mannered enough to criticize me for it.

I’ll write “hap bday” because I don’t have time for extra characters with my go-go lifestyle.

Thx!