I think the anonymity of the internet simply intensifies what's already there in the individual. If they're an inherently open, honest person, then they're going to be more open, more honest, because the parts they hold back in real life can be let go further online. If they're inherently dishonest, the online world allows for less accountability.
Part of this is not seeing the people on the other side of the screen as entirely real. Intellectually, we know that they are, but there's less of a filter on internet behavior than face-to-face. An example of this is email-wars in a business environment -- educated, professional, ostensibly intelligent people will say things in email chains, business emails, that they would never consider saying in a conference room.
I've been online for over twenty-five years. In that time, I've seen numerous examples of both extremes -- from people who poor out their inner-most secrets and personal details to a chat room of people they've never been in before ... to those who've had online friends for years that they've been lying to all along.
In my opinion, the online world gets you the best-of-the-best and the worst-of-the-worst.