I always forget to like people's comments on posts, even birthday wishes. I'll go days without going on Facebook, even on my birthday. It never occurred to me that it mattered until a friend of mine asked me to like a picture of her daughter that she posted. I thought it was so odd at our age but apparently there are Facebook social rules.

I would your friend the benefit of the doubt and assume she hadn't been online. If you texted her and she didn't respond that's one thing, but on Facebook she easily could have gotten so many "happy birthdays" that she hasn't even seen them all.