oh I have the worst headache today,hope you've had a good week
I wouldn't start a conversation in this way. Start with the 'hi, how's your week going?' Then when the peerson asks how you are you reply.
But regardless, every time this person treats you how she is, it's hurting you.
A friendship needs to be reciprocal.
I would block her.
If your sat reposting random shit on Facebook I think it's safe to say your not busy.
Maybe it's not like it for you, but for me conversation/messaging is harder work than posting stuff.
I don't expect people to reply immediately- even if they are on FB they might not feel like replying at that moment, be a bit tired or whatever and that's fine. But there are limits and I block/unfriend someone when their length of lack of response or whatever seems disrespectful- for instance if they haven't bothered replying for a few months or whatever.
Everyone has their own preferences and if someone hurts you, cut ties with them. That's self-care.
Having said that, don't expect people to reply immediately/even the same day necessarily.
But the amount of effort/care put in should be mutual/equal and even out over time.