I am fed of "hmm and okay" frm her...but still she starts talking all the time.?

She is my classmate whenever we r online on fbk she starts the chat all the time. Even the last 20 times she was the one who started the chat . But after the initial "Hi and hello wassup" as we continue I get irritated as her replies are mainly "hmm and okay " to all my texts. And I have to think for a topic all the time I mean it sounds as if am taking her interview and she replies with "ok and ya" for every question of mine. I dont reply her sometimes when I get irritated but still she starts the chat next time. What does this mean? If she isnt interested in, why does she takes initiative to talk to me?