In my view, posting in an online forum does not allow any conclusion on whether a person is introvert or extrovert. People who tend to be more introvert in real life might actually feel more encouraged to post in the anonymity of online fora or chat rooms.
What about online status in social sites like Facebook? If a person is online at the site, does that mean the person is looking for a chat partner, and is an extrovert person? I have some few Japanese friends in Facebook, but they are not online much these days. Does that mean they are an introvert type of person?
As a generality Japanese people as a whole stay pretty busy. If you have friends that are Japanese and you want to talk to them I would suggest reaching out directly if you can do so. I know I'm much more likely to answer the phone if it rings than I am to check in on a message on Facebook.
I'd say that person just isn't into having a conversation with you and it's best to leave them alone. Try to find someone else that is willing to have a conversation with you. Maybe they are uncomfortable with saying "sorry, I dont want to talk to you" or maybe they are busy and don't have time to log into facebook often. Neither has anything to do with being an introvert or an extrovert. Plus, there are a number of outside factors that could be the cause of someone not replying to you. They are busy with work, or school (thus have little to no personal time), they are ill, they have had a family crisis, their electronic has broken.
Introvert is someone that likes to spend time with themselves more than others. Being around others makes them exhausted. Many introverts are perfectly fine talking to people online and chatting.
Extrovert is someone that finds being around a LOT of people energizing and fun.