And I don’t mean using a different persona, like, e.g., saying you’re a woman when you’re a man. I think some people “speak” the same way in chatrooms or on messageboards as they do in person. But, I assume, other people speak online in a voice they would rarely use when conversing face-to-face. It kinda reminds me of the transformation many people have when they get behind the wheel of their cars.
I find it much easier to open a conversation or contribute to one on line - the anonymity helps. Personally, I’m more confident in my words than my appearance - that is, I prefer to be judged by what I say, not how I look. That said, chances are, my on-line voice is more like the “me” that I am with old friends. The obvious difference is the lost tone-of-voice, so I have to choose my words more carefully. The things I say when I can control pitch and tempo may come across completely wrong on the screen.
But, hey:
“Hi - I’m Michelle - How do you like me so far?”
Nah. I’m as pedantic IRL as I am on-line.
(But my voice is much deeper, here.)
I definitely have a different online voice. My real voice is too annoying to listen to.
I figure I use the same “voice” when I write as when I speak. It helps that I’m a touch-typist; if I had to peck out each word I’d probably try to shorten what I say.
I think the main difference between my online voice and my spoken voice is that when I’m speaking I’m not concerned about my spelling.
I am far more careful with my words in print than I am when I speak, whether writing online or not.
I have been told by many teachers and critics that my writing style is colloquial (which is just a nice word for “breezy”). I am nevertheless more careful in writing because the written word has a permanence that the spoken word does not. Of course, were I a politician or other celebrity, I would have to be as careful with my spoken words am my written one… another reason why I am happy not to be famous.
When online, the passive voice is used by me.
I sound much different in real life.
In real life, I sound like a barely literate psychopath.
Online, I sound like a somewhat-proofread barely literate psychopath. 
I tend to be an asshole online, but I’m not that way in real life. It’s not so much by choice, just that I throw my opinions out, and I don’t ever edit them to see if they’re tactful. I have a lot more tact IRL. It’s not that I’m not nice on line at times…it’s just that a lot of times, I’m cranky 'cause it’s the end of the day, and I’m fed up with things, so I just let loose.
Jman
This would seem to be an IMHO.
Online, I speak with one voice, which is “me,” and it’s a deep basso profundo.
However, in the “real” world, I have 7 screen names, each a distinct part of my personality. And in the “real” world, I get very tired of having to type and proofread everything.
All those who are assholes online say “Aye.”
For some of you, it may be that you just get it all out of your system without the self-editing people usually do IRL. But I’m assuming (again) that some who are blunt to the point of rudeness do so fully conscious that they wouldn’t talk that way IRL and don’t care. Or DO care because they… well, have their reasons.
In short, why be an asshole online?
Heh. I guess I use a different voice on-line. I never say this IRL.
Although, thinking about it, perhaps I should.
Hah. Sure you do.
Last time we all got together to drink cocktails and jitterbug to the Ellington orchestra at the Rainbow Room and we started discussing what colors our toothbrushes were, you screamed “MPSIMS!!!” and banged us all on the head with a cartoon mallet.
I still have nightmares.
Ike, was he wearing his Moderator Hat at the time? 
Rose
I’m an idiot wherever I go. 
Yeah, but Alphagene had slipped out the tinfoil lining while manny was up at the bar shouting for more ice and gin, so who knows WHO had control of his body…
I use longer words online, and am more serious. IRL I’m very silly and cheerful. I’m also more likely to get involved in pit thread-type arguments online. I know, how often do you see me in the pit? Almost never. But IRL I am completely unconfrontational. Completely. I think I’m a little more like IRL me in #straightdope, but still not completely.
I think I’m funnier online than I am in real life, but that really depends on the audience. Cause when you’re playing to a little chat box, it certainly seems more casual than talking to that same person face-to-face, so I’m usually more relaxed and goofy.
But my “voice” even changes from forum to forum. Here, I’m still new and shy and I prefer to lurk more than anything. In another forum far away from here, I’m well known and more prone to being silly or snide, depending on the situation. Here, I like to blend into the background.
PS. Did I hear jitterbugging?
I am the same IRL as I am online, including lack of tolerance for BS. My friends (few and far between as they may be), however, can hear my tone of voice, and know the difference between me ACTING like a bitch and me BEING a bitch. It’s a pretty fine distinction, actually.