Person and persona, and riding the line.
I think everyone online has an aspect of "persona" to them, if only because ideally, on the internet, you have the opportunity to think before you press "submit." Not everyone does, but the option is still there, for all of us. We filter out certain aspects of ourselves: the faces we present to the world are not exactly one-to-one identical to the faces we present in private. I'm a little wittier on the internet, because I never have to deal with l'esprit d'escalier. On the internet, it doesn't matter that I can't pronounce l'esprit d'escalier (my French pronunciation is so bad it's comical).
I swear a little less on the internet, because I have to think about the process of typing out the word. "Shut your fucking face, you fucking fucker" rolls trippingly off the tongue, but it doesn't fall quite so easy from the fingers. I don't usually document how many times I need to pee. And yeah, since I come from the "do not air your dirty laundry in public" school of thought, I can come off as a bit of a perpetual Marilyn Munster when I really tend to flux between being a Marilyn and being a Wednesday. I let my cynicism off the leash sometimes, but I've found that it's more effective when I don't live and breathe in a haze of grumpy.
Also, I really am inappropriately enthusiastic about everything. Soda. Movies. Commercials that I really like. Street pennies. Peeing. I love peeing! I mean, I don't pee on trees or anything, but I really like it when I go into the bathroom feeling uncomfortable, and come out feeling a-okay. Plus it's an excuse to sit and read, and who doesn't love that? People who are around me in the real world are likely to get treated to a constant stream of alternatingly perky and snarlingly homicidal sound bytes. "Gosh, trees are nice, I like trees I WILL DESTROY ALL WHO THWART ME do you think maybe we should go back to Disneyland in October SOMEONE ON THE INTERNET IS WRONG RARRRRRHGHGHGHGH oh hey juice." Most of these things never make it online, because they're fleeting impulses, or because I don't feel like providing an ocean of context to make them make sense.
I guess that's really where internet persona comes in, at least for me: I make more sense online. I have less visible downtime, I'm a little less random, and I'm a little more measured with my swearing. I'm just as perky, and just as cranky, it's just not a twenty-four/seven thing. It's really important to me that I not be artificial online, because I spend so much time interacting with people offline, and I don't want to be reading from a script every time I do a public appearance. (Although that would be hysterical. I should write a "being Seanan at a book signing script," and start tapping people to stand in for me while I go to get myself another soda.) Filtered doesn't mean shallow, and thoughtful doesn't mean fake.
On the balance of things, I think you can tell whether or not you'd like me in person from listening to me online, as long as you remember that there's a whole third dimension offline, and that I can sometimes use that third dimension to run into traffic after red balloons, or produce seemingly random frogs. And I find that pretty cool.
(*Who will not be named here, you know the drill, and everyone has the right to an opinion.)