Something I think is worth noting is that I also do not mimic people that I dislike, but if I become close to someone, and really, really like them, that also makes me stop mimicking. There's some kind of middle range of liking and familiarity where I mimic.
I also do not mimic at all online. I neither do it willfully nor impulsively. However, I think that's because I find more ease and security in talking online anyway. Online, I don't do my alternative mode of behavior either. I'm very different here.