1. Posting a lot of messages in forums, especially unmeaningful ones? spamming, flooding
2. Only browsing posts and threads without replying. lurking, meandering
3. Posting very long message(s) so that the whole page is his/her message? hogging (assuming the only reason to post such a long reply is to make the whole page his/her own) rambl
As for the third one, I think "hogging" is the closest.
I have attached a picture here to show you what "hogging" is. But due to the size limit of the picture, it might be not big enough to be called "hogging".
Sometimes if you think a thread is very good and worth reading, but you don't want to type much. You just type some very short words or signs, then the thread will be moved up to the top.