If it's bothering you OP, then next time she sends a 'hi, how are you' message just don't respond if it's not convenient for you.
I think having to respond really depends on the context of the message. If it's important, or I haven't spoken to someone in ages - of course I will.
But if it's just random crap they are sending me, and I'm busy then I won't bother. Thankfully my friends don't seem to care, and I wouldn't judge them either.
Having social media on your phone can be incredibly intrusive, I don't think extraverted people quite understand how draining it can be for some of us who don't want to be contacted all the time, or monitored as to how long it takes us to reply.
I would find it irritating to be expected to respond to every message, particularly as one of my friends (whom I adore and speak to regularly) will send me photos of what she is doing throughout the day - her food, her plants etc.
I used to respond with a short message or emoji, but now I only do if I have the time/ mental energy. If she has a genuine question or some news, that's different, but it bothers me that I'm expected to give my attention all day long to her.
Even though like I said we are brilliant friends who have known each-other for years. We just use our phones differently - I need to switch off more, whereas she lives on her phone.
People are just different, you may need to adjust your expectations, or tell her it bothers you.