Or else you have to be really really bad at acting, and then there'll be less of a chance that you intentionally misrepresent yourself online. Who you think you are may or may not reflect who you actually are- more likely it reflects the qualities you see as the most vital to your "you-ness." But if you're actively promoting these qualities, it's probably at the sacrifice of all those subconscious characteristics that you present throughout your day. I can try to paint a decent picture of myself on Hubski, but the very act of trying to paint the picture makes it less accurate. Might just be that the person least likely to give an accurate representation of you is you. For my part, I wish I was less conscious about how I present myself. I think I'd get my personality across a whole lot clearer that way.
Yeah I'll be honest a lot of what you said didn't occur to me at all writing that last comment, perhaps because I was so hung up on what my writing looked like :P AFK people engage in lots of different kinds of discussions, small talk, deep conversations, random stupid riffing. You get different communities online that are biased towards different kinds of communicating but I think you need to be exposed to a variety of them to really get a good idea of someone's personality. That said, looking at peoples' comments it's pretty easy to see who puts thought into their contributions and who doesn't. Positive signs would be things like including relevant hyperlinks, backing up opinions while negative signs would be things like using memes as stock responses, pedantry. I'm sure a list of them exists somewhere. So it could be that those superficial indicators have more impact on whether people think you're "painting a decent picture" of yourself online. I find that to be a pretty comforting thought.