I believe we are often actors in our real lives as well. A person who has a job as a cook, acts as a cook. He might say or do thinks he would not do at home, where he acts as father, for his 6 year old daughter. She acts as a student when she goes to school, and acts as a younger sibling when argueing with her older brother. This older brother, being 15 years old, acts as a rebellious teen.
I guess the question would be what the fundemental base is of personality and life phases. And to sketch a clear line between those. It may not be pure acting in the tradional sense, but we're still acting in a certain way. The internet is actually good evidence for this, as the majority has more space to act stupidly on the internet, because it's anonymous. It's also a welcoming place for those "stupid thoughts" that everyone has once in a while, and keep to themselves in real life.
As a result, this acting takes a more prevelant role on the forum here, or any forum for that matter. Theoratically speaking. As you say, here you have the user names as well that can add to the acting. Of course the "act" focusses more, and only, on what we choose to share here. I might act like a whiner by posting many threads where i whine, but in real life i might be a balanced person.