its totally fine to do this - as you say, in moderation
if she always asks you to go on messenger - then go on messenger! but only SOME of the times she asks. if you are actually busy, or you don't feel like it at that minute then don't go on it, or if you're feeling like you don't have anything interesting to say that day, then don't go on. just go on some of the time - and when you go on make sure any chat you have is interesting, and the minute it feels like a conversation has reached its natural end, then say i have to go now. don't worry too much if she leaves before you do, just make sure that all the conversation is fun and positive, and the minute you think its kind of going to run out of steam that day, then draw it to a close and let her know you have to go - all conversations have natural conclusions and finishing points, too many dudes want to drag it on past that point, and thats where it goes wrong for them