How come nobody ever thinks these look accurate? Every time this picture is posted, people always envision only what the worst possible user of that particular website would look like and don't try to analyze why the artist went with the designs in question.
Facebook is just a normal social networking site; no ground-breaking features or things that set it apart from everything else. Hence, normal looking college guy.
Twitter is the newest of the bunch, something of a mix between a social networking site and a chatroom; people are constantly "tweeting" each other ad nauseum. That's why the Twitter kid is the youngest looking, and looks like he could talk your ear off.
While I admit, there are some people who have used Myspace for unpure, corrupt purposes, the majority of Myspace users are just average high-school-and-college-aged preppy type kids. Most people only get Myspace accounts so they can connect with friends they already have, on Myspace. The headphones and smug expression kind of symbolize how Myspace is seen as more popular and "trendy" than Facebook, and I could easily see the Myspace and Facebook characters being rivals. The character could also equally well as a girl, though the relationship with the Facebook character would be slightly different (love-hate, maybe?).
No one's really complained about Wikipedia, and I can see why: looks super-intelligent, the kid you go to for all your hoework answers, looks serious about his education and could easily be a professor-in-training. For some reason, the green sweater is really fitting.
The Deviantart character is definately the most contested one. Firstly, I think making the character female was a good choice, since women are usually seen as more artistic than men, and the clothing colors really fit. What people complain about is how she doesn't look "depraved" enough; you really have to consider the target demographic the website creators intended the website for. Look at the character's physical appearance; mesh top and jumper, cap with rainbow gradient and little picture on the front, purple hair, etc. Her design is all about colors, textures, and gradients, key factors in creating and observing art. She looks like an individualist, she sets her own styles and doesn't go by what's popular, and is very unpredictable. I think this character captures the eccentricity of Deviantart just enough without going into the boundaries of depravity and hideousness. I mean, there are some horrible people on DA, but there are horrible people on all websites. The problem with how people perceive DA is that they let that ugliness be the only impression they have of the site, a trait that is shared with the perception of Myspace.
It took me a while to figure out the meaning of the YouTube character, but from what I have gathered, the character symbolizes being uninhibited and uncensored (while ironically being censored himself). This is fitting since, over the years, many people have turned to YouTube as an alternative to conventional TV shows, due to it being more "edgy" and, for the most part, uncensored. It is also a medium for people to freely express their opinions to anyone willing to tune in (like how the character is "expressing" his opinion of the class to Google). His and DA's clothes are actually my favorite outfits of the group.
Google, when compared to the other websites, can be seen as more business-oriented. It's layout is simple, yet iconic. It is usually the first place people go to when they want to research something on the web. Appropriately, the character is a neatly-dressed business-oriented professor. He can be seen as having seniority over the other characters since the website is used in a more professional manner compared to the others. He is a professor because he is the dispenser of knowledge, which is the same function as a search engine.
So to me, all the characters are fitting, and I think the artist did a good job with this personification. There are just some subtle details people usually don't pick up on that cause them to challenge its accurateness.