Hmm. I'm an introvert, and I find I'm less likely to participate in online discussions, mostly for the reason cycloptic mentioned above: online discussions will be read by more people and require extra care (which can be emotionally exhausting, especially on a contentious topic). I generally prefer to interact with fewer people, so one-on-one communication or small groups are ideal. You may be conflating introversion with shyness?
I tend to mix introversion with having an overly active internal thinking process.
I'm one of those people who pause before responding, because I'm thinking through my response before speaking, rather than blurting out the first thing that comes to mind.
I find overthinking a response can also be inhibiting as I can fall into the "do I really need to say that...and now", so I don't.
