I remember these sort of people from back in the heyday of MSN Messenger. They basically just start up LOADS of conversations with lots of different people at the same time, like a bulk message of "Hey" and carries the most interesting conversations, which explains your 6 hour talks on some days and then again, 1 line convos on others.
My friend used to do that, essentially really easy way to become popular. But then again I find find it annoying having loads of messages flying at me constantly, as I have a busy job and short breaks.
Quite honestly, just ignore her messages, but make sure you note them as "Seen" (Facebook/Whatsapp), then maybe she will either stop or confront you about ignoring her, in which has you can reply with "Missed me?"