I just find the fact that OP is monitoring who she replies to or not.... well, that to me shows that the issue is most likely more with the OP.

Everyone wants to feel included - but unless you're checking and rechecking someone's page you would never know that they replied to everyone but you and three others. I'd certainly never notice!

And wanting lots of friends? That's some people's goals, but not others. I go through and purge mine... I like to keep it a small number, only people I actually interact with. Why does it matter what number of people I have on it?

I just think that getting upset enough that someone didn't like their simple happy birthday message... well.... I personally find it petty to want to unfriend them for that reason only. Having someone not click "like" or thank you.... that's not extreme. It's a very little thing. It might be different if it was someone's best friend who ignored them.