I don't think it's annoying. It just becomes a habit for some of us. I use lol whenever I think my text sounded harsh. I would say "My that sounded pretty harsh. I better put lol at the end so they wouldn't get offended." That's the kind of habit I use.
I find it equally annoying. I know a girl who's facebook posts consist of some random sentence and then lolololololol. It's like, really??? Are you really lauging out loud over and over and over again?