If I may interject, a personal anecdote. "You" is general, since multiple people expressed this assertion.
You can change minds in an Internet discussion with politeness and civility.
Mafialligator actually convinced me to reassess how I use the terms "girl," "woman," etc. and as a consequence I've been more careful about those terms, to the point that I actually got into a discussion with officers of the feminist club at LSU (my roommate is one), whereupon I was informed that, though appreciated, they're not especially bothered by the use of 'girl' to describe a college-aged female (their rationale being that they considered 'guy' as a description of a college-aged male equivalent to 'boy' - essentially, college undergrads view themselves as adults, sure, but young adults, in contrast to professional working men and women, and thus are more lax about the lines). I still make the distinction anyway out of respect and belief it's right, but the point is, I've improved on it a LOT compared to when it was first brought to my attention. I've also incorporated some ideas behind critical theory of race and sex into my political philosophy, something that would never have been the case 1-2 years ago.
Now, obviously my story is but one example. I'd still say it's correct that most of the times Internet discussions don't impact the views of their participants if their participants have given a certain amount of thought to their views. There is a chance, however that a polite and civil discussion on subjects can lead to others changing their minds, where lacking politeness and civility will not.
And to be clear, you have no obligation to be polite and civil if you think someone is being sufficiently hateful to warrant it. If you don't think the chance mentioned before is worth your time then carry on as is. No one can tell you what you should do. But I would like to think that politeness, civility, and reason can make otherwise-meaningless conversations have some use after all.