Like some people have said, I can more clearly express my thoughts online. In an actual conversation I tend to sound a little less... put together and I get much more scatter-brained. However, I enjoy IRL socializing when I get the chance... I just have to be around people who are little more goofy than most because I tend to mask my awkwardness with playfulness which I've noticed can be a turn-off to some people. They end up looking at me like I'm on crack. I try to just shrug and keep going. I have fun playing around being goofy.
So back to the original point... I seem less "air-headed" and flaky online.
I've surprised many an IRL friend by randomly getting into a serious conversation after they've known me for a while and they say "Oh wow, I had no idea you were smart."