I think a lot of that type of fan support (lots of young, very vocal fans dominating the space) initially has a lot to do with couples they can ship together (see also Gio and Ranvir, Gio and Rose, Joe and Dianne etc), and now they've moved on to just supporting AJ. No disrespect to AJ as she's very charismatic so I understand why she has fans in her own right too, but just I think this partly explains the intensity of it all
I think insta followers are a fairly good indication of how much support celebs have picked up during the series (though not complete picture). From the finalists, it looks like AJ and John have picked up about 150k but Rose about 500k
(just based on my vague recollections of what they had when were announced so might not be that accurate!)